Fix sc.pl.highest_expr_genes with a categorical column

[flying-sheep]

• [x] Closes #3158
• [x] Tests included or not required because:
• [ ] Release notes not necessary because:

[codecov[bot]]

Codecov Report

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

Project coverage is 77.21%. Comparing base (f0b8d6b) to head (c0acca0). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3302 +/- ## ======================================= Coverage 77.21% 77.21% ======================================= Files 111 111 Lines 12594 12594 ======================================= Hits 9724 9724 Misses 2870 2870 ``` | [Files with missing lines](https://app.codecov.io/gh/scverse/scanpy/pull/3302?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse) | Coverage Δ | | |---|---|---| | [src/scanpy/plotting/\_qc.py](https://app.codecov.io/gh/scverse/scanpy/pull/3302?src=pr&el=tree&filepath=src%2Fscanpy%2Fplotting%2F_qc.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scverse#diff-c3JjL3NjYW5weS9wbG90dGluZy9fcWMucHk=) | `86.84% <ø> (ø)` | |