Resolve "Skip running RuboCop if there's no existing changed files"

What does this MR do and why?

Skip running RuboCop if there's no existing changed files.

Testing it at: !101520 (closed)

• Case with no interesting files: https://gitlab.com/gitlab-org/gitlab/-/jobs/3197078379
• Case with one interesting file: https://gitlab.com/gitlab-org/gitlab/-/jobs/3197299599

Original broken RuboCop job: https://gitlab.com/gitlab-org/gitlab/-/jobs/3192951015

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #378409 (closed)