Recognize np.timedelta64('NaT')
```
>>> pd._libs.tslib._delta_to_nanoseconds(np.timedelta64('NaT'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/tslib.pyx", line 3154, in pandas._libs.tslib._delta_to_nanoseconds
  File "pandas/_libs/tslib.pyx", line 3162, in pandas._libs.tslib._delta_to_nanoseconds
TypeError: an integer is required

```