Don't produce an error if there are no webhooks (#48)

kustomize or operators could result in having a webhook config with no
webhooks. This shouldn't be an error.

Signed-off-by: Haiyan Meng <haiyanmeng@google.com>

Signed-off-by: Haiyan Meng <haiyanmeng@google.com>

pkg/rotator/rotator.go

@@ -308,7 +308,7 @@ func injectCertToWebhook(wh *unstructured.Unstructured, certPem []byte) error {
 		return err
 	}
 	if !found {
-		return errors.New("`webhooks` field not found in ValidatingWebhookConfiguration")
+		return nil
 	}
 	for i, h := range webhooks {
 		hook, ok := h.(map[string]interface{})