Boolean string methods on empty Series return object dtype
#### Code Sample, a copy-pastable example if possible

```python
In [7]: pd.Series([], dtype=object).str.isalnum()
Out[7]: Series([], dtype: object)
```
#### Problem description

On empty series, we return an object-dtype result, rather than bool. I might expect
this to be a boolean dtype to match the non-empty case.

#### Expected Output

```python
In [14]: pd.Series([], dtype=object).str.isalnum()
Out[14]: Series([], dtype: bool)
```