BUG: fill_value is ignored when reindexing empty series

Here's a snippet depicting the, probably, buggy behaviour:

```
In [367]: pd.Series()
Out[367]: Series([], dtype: float64)

In [368]: _367.reindex([1,2,3])
Out[368]: 
1   NaN
2   NaN
3   NaN
dtype: float64

In [369]: _367.reindex([1,2,3], fill_value=0.0)
Out[369]: 
1   NaN
2   NaN
3   NaN
dtype: float64
```

I'd expect `_369` to contain the `fill_value` instead of `NaN`.