TST: Add test for category equalness on applies (#21239)
- [x] closes #21239
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

The test checks if equalness comparisons through `df.apply()` works correctly for the categorical `dtype`, especially with `np.NaN` and `None` values.