Don't mark empty MRs as merged on push to the target branch

What does this MR do?

When we're marking MRs as closed due to a push, don't do that if they were already empty. More reasoning at https://gitlab.com/gitlab-org/gitlab-ce/issues/32800#note_32422553 and in the commit message.

Are there points in the code the reviewer needs to double check?

I don't really like this spec file, but I don't want to refactor it all now 😧

Why was this MR needed?

Creating an MR from an issue on a high-traffic repo like this one was really annoying.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32800.