fix(backend): modelToCRDTrigger was not including periodic schedule correctly

[hbelmiro]

This PR fixes the issue where CronSchedule and PeriodicSchedule structs were incorrectly evaluated as non-empty even when their fields were all nil. The issue arose because direct struct comparison with (CronSchedule{}) or (PeriodicSchedule{}) does not account for the semantics of pointer fields being nil, "" or 0.

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[mprahl]

/lgtm

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[mprahl]

/lgtm

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[HumairAK]

/approve

[mprahl]

/lgtm

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