API/BUG: empty sum with td64 NaT or timedelta(0)?
```
ser = pd.Series([], dtype="m8[ns]")

>>> ser.sum()
NaT
>>> pd.Index(ser).sum()
NaT
>>> ser.array.sum()
NaT

>>> ser.to_frame().sum()
0   0 days
dtype: timedelta64[ns]
```

I think zero makes more sense.  Easy fix if we have consensus.
