Ignore jobs without runners when tracking failed jobs

What does this MR do and why?

This MR changes the logic in Ci::InstanceRunnerFailedJobs.track_job? so that it ignores jobs when they reference a nil runner. This fixes scenarios like https://new-sentry.gitlab.net/organizations/gitlab/issues/344228/?project=3.

• Changelog: fixed
• EE: true

Fixes #429668 (closed)

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