Fix standalone pod crashing when checking for pod owner

What does this MR do and why?

Fix Operational container crashing when scanning a standalone pod.

How to set up and validate locally

1. Setup OCS locally

2. Create a standalone pod

   • kubectl run pingpong --image alpine:3.17.3 ping 1.1.1.1

3. Start an OCS scan in the default namespace

     container_scanning:
       cadence: '00 * * * *'
       vulnerability_report:
         namespaces:
           - default
           - kube-system

• The scan should complete successfully without crashing

Linked issue Operational container scanning pod crashes when... (gitlab-org/gitlab#415140 - closed)