Handle kubeconfig null for exec auth

[danopia]

Saw kubeconfig putting explicit nulls into the exec config:

- name: gke_...
  user:
    exec:
      apiVersion: client.authentication.k8s.io/v1beta1
      args: null
      command: gke-gcloud-auth-plugin
      env: null
      installHint: Install gke-gcloud-auth-plugin for use with kubectl by following
        https://cloud.google.com/kubernetes-engine/docs/how-to/cluster-access-for-kubectl#install_plugin
      interactiveMode: IfAvailable
      provideClusterInfo: true

Let's avoid using them.