NaT - Series[timedelta64] returns NaT

Surfaced in #19139, not present in 0.22.0:

```
tdi = pd.TimedeltaIndex(['1 day', '2 day'], name='x')
ser = pd.Series(tdi)

>>> pd.NaT - ser
NaT
```

Expected:
```
>>> pd.NaT - ser
0   NaT
1   NaT
Name: x, dtype: timedelta64[ns]
```