Remove Rugged::Repository#empty?

What does this MR do?

It removes a #empty? call to rugged, as we're migrating away from Rugged.

Why was this MR needed?

Its being called, but not through Gitaly.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• [-] Conform by the merge request performance guides
• [-] Conform by the style guides
• [-] Squashed related commits together
• [-] Internationalization required/considered

What are the relevant issue numbers?

Closes gitaly#699 (closed)