REF: Change _NULL_DESCRIPTION[datetime] to use NaT sentinel
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

xref https://github.com/data-apis/dataframe-api/pull/74#discussion_r921464588

cc @vnlitvinov @jorisvandenbossche 

no tests needed updating? > no tests needed updating?

Looks like there was no existing test so added one Thanks for the feedback everyone. For the pandas implementation, I changed our sentinel to use the integer representation of `pd.NaT`