add test for not showing default when no default is given

[b3n4kh]

Adds a test for a special case when using show_default, but no default is available resulting in [default: None].

• fixes Custom default label ignored if default is None #1732

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

I needed to force push to do some cleanup. Since you made the PR from your master branch, you'll need to reset it to be in sync with ours.

git fetch upstream
git reset --hard upstream/master
git push -f origin master