Do not set PatchType if patch is empty

In admission v1, API server requires that Patch and PatchType are both
provided or none are provided.  Meanwhile, admission v1beta1 does not
have this kind of requirement.

In controller-runtime, PatchResponseFromRaw sets PatchType regardless
of the existence of patch.  If patch is empty, a response contains
only PatchType and API server does not like it.  Webhook call fails.

This change fixes this issue by not setting PatchType if patch is
empty.

pkg/webhook/admission/response.go

@@ -90,8 +90,14 @@ func PatchResponseFromRaw(original, current []byte) Response {
 	return Response{
 		Patches: patches,
 		AdmissionResponse: admissionv1.AdmissionResponse{
-			Allowed:   true,
-			PatchType: func() *admissionv1.PatchType { pt := admissionv1.PatchTypeJSONPatch; return &pt }(),
+			Allowed: true,
+			PatchType: func() *admissionv1.PatchType {
+				if len(patches) == 0 {
+					return nil
+				}
+				pt := admissionv1.PatchTypeJSONPatch
+				return &pt
+			}(),
 		},
 	}
 }