[stubgen] Fix a few bugs with stubgen of c modules

[chadrik]

Description

This fixes three issues encountered while trying to generate stubs from a c module:

1. Fix bug when inferring signatures with no args. In this case, the signature has one arg which has an empty name.
2. Add a self arg to the inferred signature for a method, when it is omitted from docstring signature. stubgen has a feature that will read the signature from the function's docstrings, but it would happily accept methods that did not specify a self-arg.
3. Use 'cls' as self arg for classmethods without signature info. Simple fix to produce better stubs for classmethods defined in c modules.

Test Plan

Each of these changes has a test which clearly shows the change.

I've left each of these changes as their own commit, and each could be a separate MR, but they are so simple I've left them in one MR. Let me know if you want me to break this up.