Resolve "Geo: Checksum failures often due to repository not existing on disk"

What does this MR do?

Returns a dummy checksum when there is no repository on disk.

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
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5576 (moved)

Sign-off

• Release manager: @rspeicher

• Engineering lead: @DouweM

• Engineering director: @tommy.morgan