check if the GeoNode table exists before checking its state

What does this MR do?

Check whether the GeoNode table exists and is connected in the GitLab::Geo.enabled? call, otherwise it will throw an error if the table does not exist.

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

Why was this MR needed?

We're getting log entries in various places when the geo_nodes table does not exist, but we're checking for a primary or secondary instance. Now check in a common place for that condition.

Screenshots (if relevant)

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

What are the relevant issue numbers?

Closes #3334 (closed)