Check for an empty value in validateField

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

Fixes #20627

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.