Fix invalid failure event for generate policy

[realshuting]

This PR

1. fixes invalid failure events for generate policy:

 ✗ k get event | grep PolicyFailed
42s         Warning   PolicyFailed      namespace/t1        policy add-ns-quota failed to apply: <nil>

2. avoids generating "non-applicable " generateRequest for the policy that does not apply:

✗ k get event | grep PolicyFailed
16s         Warning   PolicyFailed      namespace/t1        policy add-networkpolicy failed to apply: generate request gr-nnqqn: policy add-networkpolicy does not apply to resource { Namespace  t1}


✗ k get gr -n kyverno
NAME       POLICY              RESOURCEKIND   RESOURCENAME   RESOURCENAMESPACE   STATUS      AGE
gr-nnqqn   add-networkpolicy   Namespace      t1                                             52s