Applying np.sum to empty series results in NaN
``` python
In [1]: import pandas

In [2]: pandas.__version__
Out[2]: '0.7.1'

In [3]: empty = pandas.Series([], index=[])

In [4]: import numpy as np

In [5]: np.__version__
Out[5]: '1.6.1'

In [6]: np.sum(empty)
Out[6]: nan

In [7]: np.sum(empty.values)
Out[7]: 0.0
```
