api: fix ValidatingAdmissionPolicyList json tag

[xyz-li]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Change the output when validatingadmissionpolicies is empty.
kubectl get --raw /apis/admissionregistration.k8s.io/v1/validatingadmissionpolicies

{
  "kind": "ValidatingAdmissionPolicyList",
  "apiVersion": "admissionregistration.k8s.io/v1",
  "metadata": {
    "resourceVersion": "30603"
  }
}

kubectl get --raw /api/v1/nodes

{
  "kind": "NodeList",
  "apiVersion": "v1",
  "metadata": {
    "resourceVersion": "30721"
  },
  "items": []
}

Which issue(s) this PR fixes:

Fixes #1593

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fixed a bug in the API server where empty collections of ValidatingAdmissionPolicies did not have an `items` field.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: xyz-li
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[xyz-li]

/sig api-machinery

[xyz-li]

If there is no item fields, kubectl won't treat the response as a list. Then the output is wrong.

[xyz-li]

/cc @cici37

[sftim]

Changelog suggestion

-This change make api-server not omity the field `item` of ValidatingAdmissionPolicyList when ValidatingAdmissionPolicy is empty.
+Fixed a bug in the API server where empty collections of ValidatingAdmissionPolicies did not have an `items` field.

[sftim]

BTW, the response to a list is formally a collection.

[k8s-triage-robot]

This PR may require API review.

If so, when the changes are ready, complete the pre-review checklist and request an API review.

Status of requested reviews is tracked in the API Review project.

[cici37]

/triage accepted