Series of just None not converted to NaN 

I was a little surprised by this

```
In [11]: pd.Series([])
Out[11]: Series([], dtype: float64)

In [12]: pd.Series([None])  # should be NaN / float ?
Out[12]:
0    None
dtype: object

In [13]: pd.Series([None, 1])
Out[13]:
0   NaN
1     1
dtype: float64
```
