Resolve "pipeline_schedules pages throwing error 500"

What does this MR do?

Fixes the "pipeline_schedules pages throwing error 500" with the following solutions.

• Add a guard clause at the point rendering a path using pipeline_schedule.ref.
• Add a test to reproduce this case.

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

Is this proper solution from FE dev insight?

Why was this MR needed?

• People upgraded from 9.1.x to 9.2.x can't access the pipeline schedule page (Only when they registered the scheduled pipeline with nullified ref). This should be resolved.

Screenshots (if relevant)

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
• 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 #32790 (closed)