Fix a wrong MR status when merge_when_build_succeeds & project.only_allow_merge_if_build_succeeds are true

What does this MR do?

Fix a wrong MR status when merge_when_build_succeeds & project.only_allow_merge_if_build_succeeds are true.

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

@stanhu I reused your proposal from the issue, I think it's a good enough solution.

What are the relevant issue numbers?

Fixes #19035 (closed).

Does this MR meet the acceptance criteria?

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