Allowing None to parse

[jamesbraza]

As of docstring_parser==0.16, parse is actually safe to being input None as the text.

Can we type hint it to allow this? That way, parse can be invoked on functions that don't have a __doc__ attribute:

from docstring_parser import parse

def foo():
    pass

parse(foo.__doc__)  # This is runtime safe, just needs to be type safe