Inconsistent Behavior of min on Timestamps w/ and w/o timezones
related #4147

Observe w/o tz:

```
In [46]: pd.Series([pd.NaT, pd.Timestamp("2013-1-1")]).min()
Out[46]: Timestamp('2013-01-01 00:00:00', tz=None)
```

however w/ tz:

```
In [48]: pd.Series([pd.NaT, pd.Timestamp("2013-1-1", tz="US/Eastern")]).min()
Out[48]: NaT
```

strangely, `max` works:

```
In [60]: pd.Series([pd.NaT, pd.Timestamp("2013-1-1", tz="US/Eastern")]).max()
Out[60]: Timestamp('2013-01-01 00:00:00-0500', tz='US/Eastern')
```
