Fix Sidecar without image specification issue

[ovnozdrach]

This is to address issue #2966

It's possible to add a sidecar specification without providing its image.
This leads to a cascading failure where all cluster instances are shut down sequentially during sync intervals (30 minutes) until all pods are terminated, resulting in complete PostgreSQL unavailability

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[mikkeloscar]

Likely is also mitigated via schema update from: #3007 but good to have it covered here in code as well.

[mikkeloscar]

👍

[FxKu]

👍