"IndexingError: Unalignable boolean Series key provided" when passing an empty object Series
For some reason, with 0.13, an empty object Series is interpreted as a boolean one, when using to fetch elements in another Series:

```
pandas.Series(['A', 'B'])[pandas.Series([], dtype=object)]
```

The following works though, so the dtype confusion seems to be confined to empty Series:

```
pandas.Series(['A', 'B'])[pandas.Series(['C'], dtype=object)]
```
