Antrea-native policies stayed in realizing status in some cases

[tnqn]

Describe the bug

When no-op changes are applied to Antrea-native policies, such as adding or removing a non-existing group, there will be no datapath change but the generation has changed.

status:
  currentNodesRealized: 1
  desiredNodesRealized: 2
  observedGeneration: 4
  phase: Realizing

To Reproduce

1. Create the following policy:

apiVersion: crd.antrea.io/v1alpha1
kind: NetworkPolicy
metadata:
  name: allow-client-to-server
spec:
  priority: 1
  ingress:
    - action: Drop
      appliedTo:
      - podSelector: {}
      - group: abc # non-existing group
      name: abc

2. Then apply the following one which removes the group from appliedTo:

apiVersion: crd.antrea.io/v1alpha1
kind: NetworkPolicy
metadata:
  name: allow-client-to-server
spec:
  priority: 1
  ingress:
    - action: Drop
      appliedTo:
      - podSelector: {}
      name: abc

3. The policy will stay in "realizing"

Versions:

• Antrea version (Docker image tag). ~v1.12.0