Fix a NoMethodError in Geo::MetricsUpdateService

What does this MR do?

Fixes a NoMethodError on nil in Geo::MetricsUpdateService

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

Why was this MR needed?

Observed in production: https://sentry.gitlap.com/gitlab/geo1/issues/113314/

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• 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?