Fix help output for Shiv and PEX

[harkabeeparolus]

This change ensures that "Usage" help output is correctly displayed when Click scripts are run inside Shiv or PEX zipapps, where __package__ is set to the empty string rather than None.

• fixes #2332
• Problem introduced by #1609

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.