BUG(?): Var of Timedelta with empty / NA
*edit*: it seems like the bug is that var returns numeric values for the operation, instead of timedeltas like `.std` (https://github.com/pandas-dev/pandas/issues/18880#issuecomment-353164903)

---

Working on the empty / all-NA stuff. This is related, but separate.

Currently for an all-NA series of timedelta's, we return `NaT` for `.var()` on an empty series (see Out[44]). I think this should be `NaN` since the reduction is returning numeric values, not timedeltas.

```python
In [40]: s = pd.Series(pd.timedelta_range(0, periods=12, freq='S'))

In [41]: s[0] = np.nan

In [42]: s.var()
Out[42]: 1.1e+19

In [43]: s.var(skipna=False)
Out[43]: nan

In [44]: s[:0].var()
Out[44]: NaT
```

I've just noticed some other buggy behavior with aggregations on timedeltas, so maybe this can become a meta-issue about timedeltas and numeric aggregations:

```python
In [57]: s.sum(skipna=False)
Out[57]: Timedelta('-106752 days +00:13:49.145223')
```

I'm guessing we improperly add the timedelta min here.