Empty repository mirror no longer creates master branch with README automatically

Why was this MR needed?

Before, if the project had an empty repository (no branches, no files) in the first mirror it would pull and create a master branch with a README file automatically. This is not expected behaviour since it will make the repository diverge from upstream.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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 #2425 (closed)