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

[zalando-robot]

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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]

👍

[ovnozdrach]

Hi @FxKu ! Looks like the change was mistakenely mapped to #2997 in the Release notes . Maybe it's worth to correct that?
- prevent sidecars without image specification to not run into cascading failures (https://github.com/zalando/postgres-operator/pull/2997)