BUG: read_csv not replacing nans before date conversion
- [x] closes #11019 
- [x] closes #26203
- [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

Not a clue how this could influence mypy in read_xml Ah it is the init file...