Differentiate between a type being null and undefined/None

[stufisher]

The changes in https://github.com/MarcoMuellner/openapi-python-generator/pull/53 seem to break genuine any type definitions. I'm not an openapi expert but swagger tells me that if type is not defined, i.e. undefined/None then this is considered any. The other case where it is defined as null should be correctly translated as None.

I split the branch to allow genuine anys.

[MarcoMuellner]

Hey there, thanks for your contribution. I'll gladly merge this