Fix first_valid_index() for Empty object

[itholic]

Series.first_valid_index() and DataFrame.first_valid_index() raise Exception for Empty set - but it supposed to return None, not raise Error. -

>>> ks.Series([]).first_valid_index()  # It should return `None`.
Traceback (most recent call last):
...
TypeError: 'NoneType' object is not subscriptable

This PR fixed this and added related tests.

>>> ks.Series([]).first_valid_index() is None
True

[ueshin]

Thanks! merging.