Bug: Error printing stats starting at Sortino Ratio

### Expected behavior

Hello,

For many values of my strategies I have to limit my stats ouptut to the first 12 lines `stats.head(12`)` or I get an error. I guess this is related to `Sortino Ratio` display which is the next row which should appear. Maybe it can't be computed for some input values and its output is 'Ǹan'
I have no problem with plotting
I can send you the code whether required.

Thanks for your help

### Code sample

```python

```

### Actual behavior

```
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:770, in PlainTextFormatter.__call__(self, obj)
[763](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:763) stream = StringIO()
[764](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:764) printer = pretty.RepresentationPrinter(stream, self.verbose,
[765](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:765) self.max_width, self.newline,
[766](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:766) max_seq_length=self.max_seq_length,
[767](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:767) singleton_pprinters=self.singleton_printers,
[768](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:768) type_pprinters=self.type_printers,
[769](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:769) deferred_pprinters=self.deferred_printers)
--> [770](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:770) printer.pretty(obj)
[771](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:771) printer.flush()
[772](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/core/formatters.py:772) return stream.getvalue()

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:419, in RepresentationPrinter.pretty(self, obj)
[408](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:408) return meth(obj, self, cycle)
[409](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:409) if (
[410](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:410) cls is not object
[411](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:411) # check if cls defines __repr__
(...)
[417](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:417) and callable(_safe_getattr(cls, "__repr__", None))
[418](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:418) ):
--> [419](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:419) return _repr_pprint(obj, self, cycle)
[421](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:421) return _default_pprint(obj, self, cycle)
[422](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:422) finally:

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:794, in _repr_pprint(obj, p, cycle)
[792](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:792) """A pprint that just redirects to the normal repr function."""
[793](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:793) # Find newlines and replace them with p.break_()
--> [794](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:794) output = repr(obj)
[795](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:795) lines = output.splitlines()
[796](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/IPython/lib/pretty.py:796) with p.group():

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:196, in _Stats.__repr__(self)
[189](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:189) def __repr__(self):
[190](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:190) with pd.option_context(
[191](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:191) 'display.max_colwidth', 20, # Prevent expansion due to _equity and _trades dfs
[192](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:192) 'display.max_rows', len(self), # Reveal self whole
[193](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:193) 'display.precision', 5, # Enough for my eyes at least
[194](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:194) # 'format.na_rep', '--', # TODO: Enable once it works
[195](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:195) ):
--> [196](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/backtesting/_stats.py:196) return super().__repr__()

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1784, in Series.__repr__(self)
[1782](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1782) # pylint: disable=invalid-repr-returned
[1783](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1783) repr_params = fmt.get_series_repr_params()
-> [1784](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1784) return self.to_string(**repr_params)

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1883, in Series.to_string(self, buf, na_rep, float_format, header, index, length, dtype, name, max_rows, min_rows)
[1831](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1831) """
[1832](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1832) Render a string representation of the Series.
[1833](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1833)
(...)
[1869](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1869) '0 1\\n1 2\\n2 3'
[1870](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1870) """
[1871](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1871) formatter = fmt.SeriesFormatter(
[1872](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1872) self,
[1873](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1873) name=name,
(...)
[1881](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1881) max_rows=max_rows,
[1882](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1882) )
-> [1883](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1883) result = formatter.to_string()
[1885](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1885) # catch contract violations
[1886](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/core/series.py:1886) if not isinstance(result, str):

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:320, in SeriesFormatter.to_string(self)
[318](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:318) else:
[319](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:319) fmt_index = index._format_flat(include_name=True)
--> [320](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:320) fmt_values = self._get_formatted_values()
[322](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:322) if self.is_truncated_vertically:
[323](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:323) n_header_rows = 0

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:297, in SeriesFormatter._get_formatted_values(self)
[296](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:296) def _get_formatted_values(self) -> list[str]:
--> [297](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:297) return format_array(
[298](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:298) self.tr_series._values,
[299](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:299) None,
[300](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:300) float_format=self.float_format,
[301](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:301) na_rep=self.na_rep,
[302](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:302) leading_space=self.index,
[303](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:303) )

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1161, in format_array(values, formatter, float_format, na_rep, digits, space, justify, decimal, leading_space, quoting, fallback_formatter)
[1145](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1145) digits = get_option("display.precision")
[1147](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1147) fmt_obj = fmt_klass(
[1148](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1148) values,
[1149](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1149) digits=digits,
(...)
[1158](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1158) fallback_formatter=fallback_formatter,
[1159](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1159) )
-> [1161](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1161) return fmt_obj.get_result()

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1195, in _GenericArrayFormatter.get_result(self)
[1193](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1193) def get_result(self) -> list[str]:
[1194](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1194) fmt_values = self._format_strings()
-> [1195](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1195) return _make_fixed_width(fmt_values, self.justify)

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1740, in _make_fixed_width(strings, justify, minimum, adj)
[1737](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1737) else:
[1738](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1738) adjustment = adj
-> [1740](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1740) max_len = max(adjustment.len(x) for x in strings)
[1742](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1742) if minimum is not None:
[1743](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1743) max_len = max(minimum, max_len)

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1740, in (.0)
[1737](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1737) else:
[1738](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1738) adjustment = adj
-> [1740](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1740) max_len = max(adjustment.len(x) for x in strings)
[1742](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1742) if minimum is not None:
[1743](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/format.py:1743) max_len = max(minimum, max_len)

File /opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/printing.py:511, in _TextAdjustment.len(self, text)
[510](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/printing.py:510) def len(self, text: str) -> int:
--> [511](https://file+.vscode-resource.vscode-cdn.net/opt/miniconda3/envs/quants-lab/lib/python3.10/site-packages/pandas/io/formats/printing.py:511) return len(text)

TypeError: object of type 'NoneType' has no len()
```

### Additional info, steps to reproduce, full crash traceback, screenshots

_No response_

### Software versions

- backtesting 0.6.3
- Debian 12

Start in backtesting/_stats.py at _Stats.__repr__ and reproduce the failure when displaying stats beyond the first 12 rows, especially around Sortino Ratio. Inspect how the Series is rendered through pandas and compare inputs that produce the reported NaN output. Done means full stats display completes without the TypeError for the affected strategy values.

Written by the indexing model from the issue text.