Empty default value in help output

[pbeza]

This PR solves the problem with printing empty default values in --help output when @click.option has multiple=True set.

• fixes Empty default value in --help output for option with multiple=True #1969

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.