Removed adding a space into empty docstrings. (#2249)

Resolves #2168 by disabling the insertion of a " " when the docstring is entirely empty.

Note that this PR is focussed only on the case of empty docstrings. In particular this does not make any changes to the behaviour that a " " is inserted if a non-empty docstring begins with the quoting character. That is, black still prefers:

    """ "something" """

to:

    """"something" """

and that:

    """"Something""""

is not a legal docstring.