Fix case __getitem__

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 89.63%. Comparing base (8d0ef79) to head (839faaa).

Files	Patch %	Lines
foamlib/_cases.py	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #119 +/- ## ========================================== + Coverage 89.52% 89.63% +0.11% ========================================== Files 9 9 Lines 802 801 -1 ========================================== Hits 718 718 + Misses 84 83 -1 ```

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