BUG: Index.is_monotonic... incorrectly caching Index.is_unique when first value is NaT
- [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).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] 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)
```
Thanks @lukemanley 