K8SPG-772: Fix panic if backup completedAt is nil

[egegunes]

CHANGE DESCRIPTION

Problem:
WAL watcher panics if some backups have no CompletedAt status field.

Solution:
Add a nil check and use CreationTimestamp as fallback.

CHECKLIST

Jira

• Is the Jira ticket created and referenced properly?
• Does the Jira ticket have the proper statuses for documentation (Needs Doc) and QA (Needs QA)?
• Does the Jira ticket link to the proper milestone (Fix Version field)?

Tests

• Is an E2E test/test case added for the new feature/change?
• Are unit tests added where appropriate?

Config/Logging/Testability

• Are all needed new/changed options added to default YAML files?
• Are all needed new/changed options added to the Helm Chart?
• Did we add proper logging messages for operator actions?
• Did we ensure compatibility with the previous version or cluster upgrade process?
• Does the change support oldest and newest supported PG version?
• Does the change support oldest and newest supported Kubernetes version?

[JNKPercona]

Test name	Status
backup-enable-disable	passed
custom-extensions	passed
custom-tls	passed
demand-backup	passed
finalizers	passed
init-deploy	passed
monitoring	passed
monitoring-pmm3	passed
one-pod	passed
operator-self-healing	passed
pitr	passed
scaling	passed
scheduled-backup	passed
self-healing	failure
sidecars	passed
start-from-backup	passed
tablespaces	passed
telemetry-transfer	passed
upgrade-consistency	passed
upgrade-minor	passed
users	passed
We run 21 out of 21

commit: 2293150
image: perconalab/percona-postgresql-operator:PR-1149-229315070