Inconsistent processing of NaT values by GroupBy
```python
df = pd.DataFrame({'number': [1, 2, 3], 'time': [pd.NaT, pd.NaT, pd.NaT]})
interval_index = pd.interval_range( start=pd.to_timedelta('10 days'), end=pd.to_timedelta('15 days'), freq='1h' )
df.groupby( pd.cut(df['time'], bins=interval_index) )['number'].count()
```

The example above results in NaT values being counted in the (12 days 11:00:00, 12 days 12:00:00] range. The expected result is 0.

The behavior is also inconsistent. Tweaks to the `interval_index` or the `groupby` operation result in the expected output. For example:

```
end=pd.to_timedelta('14 days')
```

or

```
df.groupby( pd.cut(df['time'], bins=interval_index) )['time'].count()
```

#### Output of ``pd.show_versions()``
<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.4.0-18362-Microsoft
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.4
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 44.0.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.15.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>