Allow empty kube_node group (#11248)

While uncommon, provisioning only a control plane is a valid use case,
so don't block it.

roles/kubernetes/preinstall/tasks/0040-verify-settings.yml

 ---
 - name: Stop if either kube_control_plane or kube_node group is empty
   assert:
-    that: "groups.get( item )"
-  with_items:
-    - kube_control_plane
-    - kube_node
+    that: groups.get( 'kube_control_plane' )
   run_once: true
   when: not ignore_assert_errors