BUG: Regresssion in handling of empty Series as indexers to Series (GH5877)

[jreback]

closes #5877

In [1]: s = Series(['A', 'B'])

In [2]: s[Series(['C'], dtype=object)]
Out[2]: 
C    NaN
dtype: object

In [3]: s[Series([], dtype=object)]
Out[3]: Series([], dtype: object)

This didn't work in 0.12

In [4]: s[Series([], dtype=bool)]
Out[4]: Series([], dtype: object)

This raises though (in both 0.12 and 0.13)

In [5]: s[Series([True], dtype=bool)]
IndexingError: Unalignable boolean Series key provided
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