Allow creating merge request even if target branch is not specified in query params

What does this MR do?

It fixes a regression where URL to the new merge request form generated when pushing to GitLab unnecessarily leads to the compare branches screen.

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?

Closes #28890 (closed)