BUG: datetime64 series reduces to nan when empty instead of nat
I ran into some strange behavior with a series of dtype datetime64[ns] where I called max and got back a `nan`. I think the correct behavior here is to return `nat`. I looked through test_nanops but I am not sure where the correct place to put the test for this is.

The new behavior is:

``` python
In [1]: pd.Series(dtype='datetime64[ns]').max()
Out[1]: NaT
```

where the old behavior was:

``` python
In [1]: pd.Series(dtype='datetime64[ns]').max()
Out[1]: nan
```
