[Direct to 5.15] Handle empty initContainers array in postgres upgrade flow

[dannyzaken]

Explain the changes

1. Adding the "missing" initContainer right before accessing podSpec.InitContainers array.
2. In the previous fix, the init container was added to the DB reconcile, which happens after the upgrade flow.

Issues: Fixed #xxx / Gap #xxx

1. https://bugzilla.redhat.com/show_bug.cgi?id=2281839

Testing Instructions:

1. install noobaa 4.14
2. stop the operator
3. remove the init container from DB sts
4. start the operator
5. upgrade to 4.15
6. before the fix, the operator would crash. after the fix, it should continue with the upgrade as expected

• Doc added/updated
• Tests added