BUG: repr of np.datetime64('NaT') in Series/DataFrame with dtype object
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Discovered while investigating #12499

Before:

```
In [1]: pd.Series([np.datetime64('NaT')], dtype=object)
Out[1]:
0    NaN
dtype: object
```
After:

```
In [1]: pd.Series([np.datetime64('NaT')], dtype=object)
Out[1]:
0    NaT
dtype: object
```