conditionally log unused annotations

[UebelAndre]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

For every use of pip_repository, a warning log will be printed about unused package_annotations when there aren't any (prints an empty list). There's no reason to print an empty list

Issue Number: N/A

What is the new behavior?

A warning will only be printed about unsued annotations if there are actually some that are unused.

Does this PR introduce a breaking change?

• Yes
• No

Other information