Ensure groups are never empty [RUN ALL RSPEC] [RUN AS-IF-FOSS]

This MR aims to help address #325693 (closed) by preventing empty groups from being returned to the client.

This MR is aimed as a temporary diagnostic measure to detect cases where the pipeline is readable by the current user, but one or more jobs is not readable.

If this makes a Ci::Group empty, then we skip it (preventing the front-end error), and report this to sentry, with sufficient information (hopefully) to triage the bug.