BUG: to_timedelta('NaT', box=False) returns datetime64('NaT')

On pandas 0.23 (correct):
```
In [2]: pd.to_timedelta('NaT', box=False)
Out[2]: numpy.timedelta64('NaT','ns')
```

But on pandas 0.24:
```
In [2]: pd.to_timedelta('NaT', box=False)
Out[2]: numpy.datetime64('NaT')
```

The result is a datetime, not a timedelta.