Series(index=DatetimeIndex(...)) constructor very slow
When instantiating a series with a datetimeindex but no data value (=None), a very slow codepath is used (involving Timestamp creation), compared to if a NaN value is supplied. The results are identical.

```
dr = pd.date_range(
    start=datetime(2015,10,26),
    end=datetime(2016,1,1),
    freq='10s'
)  # ~500k long
pd.Series(index=dr)  # slow, 2.78 s
pd.Series(np.nan, index=dr)  # fast, 0.01 s
```

pandas version 0.17.0
