FIX If the `help` of an argument is `None`, don't fail

[hoodmane]

Summary

This is a fix for a regression introduced in #134. It's possible that the argument has a help attribute, but the value of it is None. We then hit a type error where we pass None to re.sub.

Tasks

• [x] Added unit tests
• [x] Added documentation for new features (where applicable)
• [x] Added release notes (using reno)
• [x] Ran test suite and style checks and built documentation (tox)

(no release note needed since it's a fix for an unreleased feature)