Boolean string methods on empty Series return object dtype

[TomAugspurger]

Code Sample, a copy-pastable example if possible

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

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

[TomAugspurger]

Given that other boolean-returning methods do return a bool dtype for empty Series, I'm comfortable calling this a bug:

In [5]: pd.Series([], dtype=object).str.contains("a")
Out[5]: Series([], dtype: bool)

[ellequelle]

It looks to me like this can be solved by adding dtype=bool as an argument to _noarg_wrapper for the "ismethods" the StringMethods class:

isalnum = _noarg_wrapper(
    lambda x: x.isalnum(),
    name="isalnum",
    docstring=_shared_docs["ismethods"] % _doc_args["isalnum"],
    returns_string=False,
    dtype=bool,
)