BUG: Series.get with Float64Index and numpy nan returns empty Series
I tried to use series.get(x) (where x is np.nan in some instance) and the result was an empty series.  It seems like it should just return None if it can't be found.

```
import pandas as pd
import numpy as np

series = pd.Series(range(0, 10))
series.get(np.nan) 
# returns None

series = series.reindex(pd.Float64Index(range(0, 10)))

series.get(10) 
# This correctly returns None

series.get(np.nan) 
# returns empty series with empty Float64Index

series.get(np.nan, np.nan) 
# same thing
```
