BUG: Cast a key to NaT before get loc from Index
- [x] closes #13603
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

pd.NaT, None, float('nan') and np.nan are
converted NaT in TestTimedeltas.
So we should convert keys if keys are these value.

Fix #13603
