Fix TypeError in MiddleWordEm extra when options was None (#627)

[Crozzers]

This PR fixes #627, where if the extra options for middle-word-em was set to None it would raise a TypeError.

For extras, it's possible to initialise them without any options like so: extras={'abc': None}. This is usually handled in Extra.__init__ and converted to an empty dictionary, so that the extras don't have to worry about a bunch of runtime type checks:

python-markdown2/lib/markdown2.py

Line 2382 in c91007d

self.options = options if options is not None else {}

However, middle-word-em would try to set some default options before calling super().__init__, and didn't have sufficient checks for if the value is None. This PR adds a check and converts it to a dict if so

[nicholasserra]

Thank you!