Prefer nullable fields for oncall schedule types
What does this MR do?
In planning for #259172 (closed), I found that we'll actually need to return null values for some non-nullable GraphQL fields (primarily OncallParticipantType#id and OncallRotationType#id, because a preview of a rotation will not be presenting persisted objects; WIP MR: !57014 (closed)).
This MR makes all on-call schedule fields nullable - which is technically a breaking changes. However, the feature flag for on-call schedules was enabled and removed earlier %13.11.
The deprecation process for GraphQL is release-based, https://docs.gitlab.com/ee/api/graphql/#deprecation-and-removal-process, so if we make any modifications before the release cutoff, I don't think we should need to go through a deprecation for these fields.
If that's not the correct thinking or this MR doesn't get merged in time, and I'll come up with a different approach for #259172 (closed).
This MR is certainly reinforcement for defaulting to nullable fields as a best practice! It's not often an id may become null-able, but here we are!
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