Resolve "Error 500 on route "/admin/application_settings/services/1882/edit" -> edit Prometheus Service Template"

What does this MR do?

Fixes admin Prometheus configuration view

Why was this MR needed?

A configuration banner that allows cluster creation is displayed on the Prometheus configuration page. This works fine if the service is being configured from the project settings configuration, but it fails with a 500 if the service is being configured from the Admin dashboard (since there's no project).

Bug probably introduced on: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16182

Screenshots (if relevant)

From the project settings

From the admin area

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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43532 (closed)