BUG: pd.to_datetime with format doesn't work with pd.NA
- [x] closes #42957
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Hi @jreback - reverted to cython solution. Let me know if this looks good. Hi @jreback - do you mind taking a look, if this is fine? thanks @debnathshoham 