Do not create a MergeRequestDiff record when source branch is deleted

What does this MR do?

It prevents creating a MergeRequestDiff record that later causes errors described in #23413 (closed) and #23897 (closed).

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

• Is this empty MergeRequestDiff that we used to create necessary in some less obvious scenario?

Does this MR meet the acceptance criteria?

• Changelog entry added
• [ ] Documentation created/updated
• [ ] API support added
• 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?

Fixes #23413 (closed) Fixes #23897 (closed)