Fix pipeline schedules view when owner is nil

What does this MR do and why?

This backports !134041 (merged) to 16-4-stable-ee.

The pipeline schedule Vue refactor in !128913 (merged) caused pipeline schedules not to be rendered when an owner of a pipeline schedule was nil. This can happen if the user is deleted.

Update the GraphQL schema and Vue code to allow for a nil owner.

Relates to #427747 (closed)

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