Gracefully handle nil created_at values in CI pipelines

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/90027 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 https://gitlab.com/gitlab-org/gitlab/-/issues/369124

Changelog: fixed