COMPAT: should an empty string match a format (or just be NaT)
- [x] closes #13044
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (not applicable)

Seems like the failing tests are not related to my changes - @alimcmaster1 could you please verify? How should I proceed with this issue? Would rebasing on current master help? thanks @fgebhart very nice