fix npe when inventory object is nil

[chanwit]

Fixes #3902

[enekofb]

we could add a test case here for the scenario we are trying to handle

apiVersion: kustomize.toolkit.fluxcd.io/v1
kind: Kustomization
metadata:
  creationTimestamp: "2023-07-31T15:05:53Z"
  finalizers:
  - finalizers.fluxcd.io
  generation: 1
  labels:
    kustomize.toolkit.fluxcd.io/name: flux-system
    kustomize.toolkit.fluxcd.io/namespace: flux-system
  name: rbac-secrets-good-practices
  namespace: flux-system
  resourceVersion: "9437086"
  uid: f3780853-bf47-4a3b-b5e3-f029623e8595
spec:
  force: false
  interval: 1m0s
  path: ./goodpractices/kubernetes/rbac/secrets
  prune: true
  sourceRef:
    kind: GitRepository
    name: policy-library
status:
  conditions:
  - lastTransitionTime: "2023-07-31T15:05:54Z"
    message: Source is not ready, artifact not found
    observedGeneration: 1
    reason: ArtifactFailed
    status: "False"
    type: Ready
  observedGeneration: -1

added the example also to the ticket

[bigkevmcd]

@chanwit are you going to address these issues? if not, I'll happily close this and open a new PR to do it.

[chanwit]

I'll continue this week. Sorry I am currently having a technical issue with my box.