Don't fail on missing file

Nobody has claimed this yet.

In this case, the benchmarks were not set up yet on the repo, and I think it would be preferable to just pass through and not fail to create a new table from the file.

```
Traceback (most recent call last):
File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
exec(code, run_globals)
File "/home/runner/.local/lib/python3.10/site-packages/lf_asv_formatter/__main__.py", line 115, in
headers, rows = format_asv_table_from_file(output_file)
File "/home/runner/.local/lib/python3.10/site-packages/lf_asv_formatter/__main__.py", line 54, in format_asv_table_from_file
headers = format_headers(rows[0])
IndexError: list index out of range
```

Start at lf_asv_formatter/__main__.py, especially format_asv_table_from_file and the module entry point shown in the traceback. Reproduce the empty or missing benchmark-file case and verify that table creation no longer fails when no rows are available.

Written by the indexing model from the issue text.