Merge pull request #25709 from asalkeld/null-deref

Automatic merge from submit-queue

Check for an empty value in validateField

```release-note
* Fix a panic when args was not supplied with any values.
```

reflect.TypeOf() can take a nil (it then returns a nil), but
Kind() panics on a nil.

Now the user gets the following output:
./kubectl.sh --server=http://localhost:8080 create -f ../../test-files/test-rc.yaml
error validating "../../test-files/test-rc.yaml": error validating data: unexpected nil value for field spec.template.spec.containers[0].args[0]; if you choose to ignore these errors, turn validation off with --validate=false

fixes #20627 and fixes #26927

hack/testdata/invalid-rc-with-empty-args.yaml

@@ -0,0 +1,20 @@
+apiVersion: v1
+kind: ReplicationController
+metadata:
+  name: kube-dns-v10
+  namespace: kube-system
+spec:
+  replicas: 1
+  selector:
+    k8s-app: kube-dns
+  template:
+    metadata:
+      labels:
+        k8s-app: kube-dns
+    spec:
+      containers:
+      - name: carbon-relay
+        image: banno/carbon-relay
+        args:
+        -
+# above is the empty arg string.