BUG: using .replace() on a Series containing np.nan raising ValueError when using regex=True.
- [x] closes #43344
- [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

nice, thanks @DriesSchaumont 