TST: Add test for where inplace
This adds a test for ensuring that None-to-NaN type-casting is consistent with `where` when `inplace=True` and otherwise.

- [x] closes #22051 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them

thanks @rlvoyer 