quick fix for empty groupname

[eacharles]

Problem & Solution Description (including issue #)

Code Quality

• [ ] My code follows the code style of this project
• [ ] I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• [ ] My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• [ ] Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (f980ecc) 99.53% compared to head (f2f02a2) 99.53%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #84 +/- ## ======================================= Coverage 99.53% 99.53% ======================================= Files 8 8 Lines 641 641 ======================================= Hits 638 638 Misses 3 3 ``` | [Files](https://app.codecov.io/gh/LSSTDESC/tables_io/pull/84?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC) | Coverage Δ | | |---|---|---| | [src/tables\_io/ioUtils.py](https://app.codecov.io/gh/LSSTDESC/tables_io/pull/84?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LSSTDESC#diff-c3JjL3RhYmxlc19pby9pb1V0aWxzLnB5) | `100.00% <ø> (ø)` | |

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