Change NA to NaT for datetime and timedelta types

[galipremsagar]

Description

Resolves: #13867

This PR:

• Changes all repr code of datetime & timdelta types to switch to represent null values as NaT from <NA>.
• Changes Scalar getitem to return cudf.NaT instead of cudf.NA

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

This looks good overall. I'm a little surprised the changeset isn't larger than this. I would like to think about refactoring the ways we check if values are "None, NA, NaT"-like. I understand it might vary by the calling code, but we might be able to reduce repeated logic a little. If you don't see an opportunity to improve that, I am fine with accepting this PR as-is.

[galipremsagar]

This looks good overall. I'm a little surprised the changeset isn't larger than this. I would like to think about refactoring the ways we check if values are "None, NA, NaT"-like. I understand it might vary by the calling code, but we might be able to reduce repeated logic a little. If you don't see an opportunity to improve that, I am fine with accepting this PR as-is.

I was able to add is_na_like utility and replaced all such repetitive patterns.

[bdice]

Great! Thanks for the iterations.

[galipremsagar]

/merge