Avoid calling underlying methods on non-existing repos

What does this MR do?

Avoid calling underlying methods on non-existing repos, hopefully clearing up Gitaly's Sentry from the many GRPC::NotFound occurrences (e.g. https://sentry.gitlap.com/gitlab/gitaly-production/issues/105514/) and unlocking gitaly#220 (closed)

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

Is there any other method apart from #exists? that we might want to skip this optimization for?

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

What are the relevant issue numbers?

Unlocks gitaly#220 (closed)