BUG: consistent downcast on fillna no-ops
- [x] xref #45423
- [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
- [x] whatsnew entry

thanks, left the issue open if any api discussion left.