Series.__sub__(NaT) vs DatetimeIndex.__sub__(NaT)
One treats NaT result as datetime, other treats it as timedelta.

```
>>> dti  = pd.DatetimeIndex([pd.NaT, pd.Timestamp('19900315')])
>>> ser = pd.Series(dti)

>>> dti - pd.NaT
TimedeltaIndex([NaT, NaT], dtype='timedelta64[ns]', freq=None)

>>> ser - pd.NaT
0   NaT
1   NaT
dtype: datetime64[ns]
```