Show empty default

[m-freiberg]

Use None instead of "" to indicate that default_string is unset to allow for empty string defaults.

https://github.com/pallets/click/issues/2500

• fixes #2500

New behavior:

Checklist:

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