BUG: Index.is_monotonic... incorrectly caching Index.is_unique when first value is NaT

[lukemanley]

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/v2.2.0.rst file if fixing a bug or adding a new feature.

Fixes the following:

In [1]: import pandas as pd

In [2]: idx = pd.to_datetime([None, "2000-01-01", "2000-01-01"])

In [3]: idx.is_unique, idx.is_monotonic_increasing
Out[3]: (False, False)

In [4]: idx = pd.to_datetime([None, "2000-01-01", "2000-01-01"])

In [5]: idx.is_monotonic_increasing, idx.is_unique
Out[5]: (False, True)

[mroeschke]

Thanks @lukemanley