Fix "pipeline_schedules pages throwing error 500" when "Active Ci::PipelineSchedule have a nullified `next_run_at`"

What does this MR do?

This MR fixes "pipeline_schedules pages throwing error 500" which was recently reopened as regression . The reason was "Active Ci::PipelineSchedule have a nullified next_run_at" which was tracked separately, because the reproducible way was not clarified yet.

This is a quick fix. We should plan long term solution later.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Because https://gitlab.com/gitlab-org/gitlab-ce/issues/32790 is regression . Also, a customer encountered this error. (ZD: https://gitlab.zendesk.com/agent/tickets/80289)

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32790
• Related https://gitlab.com/gitlab-org/gitlab-ce/issues/35931