Geo - Fix RepositorySyncService when cannot obtain a lease to sync repository

Why was this MR needed?

Gitlab::ExclusiveLease returns false instead of nil when the lease is already taken.

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