Gracefully handle nil created_at values in CI pipelines

Merged Stan Hu requested to merge sh-fix-pipeline-age-in-minutes into master

What does this MR do and why?

!90027 (merged) added CI::Pipeline#age_in_minutes. This method causes a 500 error for any page that attempts to load an old pipeline that happens to have a nil created_at value. For these pipelines, just return 0.

Relates to #369124 (closed)

MR acceptance checklist

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Edited by Stan Hu