editgroup diff view fails on some short strings in refs, via TOML

Trying to view the "diff" on https://fatcat.wiki/editgroup/wdtofwwopvezbm2hsdt4kr3etm results in a 500 error because TOML generation fails on `release_rpoh4fe2lnhprj74yrn4pac7xa`.

The root cause seems to be trying to TOML-ify the references, which include stub metadata like `{'unstructured': '\xa0'}`.

To reproduce in python3, with the `toml` library installed:

```
import toml, json

toml.dumps(json.loads('{"unstructured": "\xa0"}'))
```

The resulting stack trace is like:

```
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
in
----> 1 toml.dumps(json.loads('{"unstructured": "\xa0"}')
2
3 )

/usr/lib/python3/dist-packages/toml/encoder.py in dumps(o, encoder)
58 if encoder is None:
59 encoder = TomlEncoder(o.__class__)
---> 60 addtoretval, sections = encoder.dump_sections(o, "")
61 retval += addtoretval
62 outer_objs = [id(o)]

/usr/lib/python3/dist-packages/toml/encoder.py in dump_sections(self, o, sup)
224 if o[section] is not None:
225 retstr += (qsection + " = " +
--> 226 unicode(self.dump_value(o[section])) + '\n')
227 elif self.preserve and isinstance(o[section], InlineTableDict):
228 retstr += (qsection + " = " +

/usr/lib/python3/dist-packages/toml/encoder.py in dump_value(self, v)
178 dump_fn = self.dump_funcs[list]
179 # Evaluate function (if it exists) else return v
--> 180 return dump_fn(v) if dump_fn is not None else self.dump_funcs[str](v)
181
182 def dump_sections(self, o, sup):

/usr/lib/python3/dist-packages/toml/encoder.py in _dump_str(v)
111 else:
112 joiner = "u00"
--> 113 v = [v[0] + joiner + v[1]] + v[2:]
114 return unicode('"' + v[0] + '"')
115

IndexError: list index out of range
```

The metadata seems bad, but it also feels like the python TOML library should not choke on this. The rust TOML library (used in `fatcat-cli`) outputs something like:

```
[[refs]]
index = 0

[refs.extra]
unstructured = " "
```