nil-check Repository::is_ancestor?

What does this MR do?

Fixes #33377 (closed) by nil-checking before calling rugged.merge_base or Gitaly::CommitIsAncestor

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

Why was this MR needed?

Because it goes 💥 in production (see related issue for sentry-links)

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 #33377 (closed)