Add support for skipping empty summaries

[technicalpickles]

Closes #530

⚡ Summary

Add empty_summary to values of skip_output for those cases when there's commands to run for a hook (ie all have been skipped, or there's no files matching command patterns)

☑️ Checklist

• Check locally
• Add tests
• Update documentation

[mrexox]

This feels good, thank you! Do you want to also update the docs? If not, I can merge the PR and do it for you :)

[technicalpickles]

@mrexox I can update the docs, I mostly wanted to get a 👍🏻 before doing it 😅

I'm not sure why the coverage is failing though. I did not think there was much in the way of tests to actually make sure the output is skipped, so I didn't have anything to crib.

[mrexox]

No problem, it's just integration tests, they are not supposed to cover all possible cases.

[technicalpickles]

Added! I didn't see much else to document except in the skip_output section.

Feel free to make any other changes needed to land, but I'm also happy to do so.

[mrexox]

Thank you a lot! I am going to close this issue before creating a new release, so I hope to release this change next week.