cmd/tailscale/cli: fix configuring partially empty kubeconfig (#11417)

When a user deletes the last cluster/user/context from their
kubeconfig via 'kubectl delete-[cluster|user|context] command,
kubectx sets the relevant field in kubeconfig to 'null'.
This was breaking our conversion logic that was assuming that the field
is either non-existant or is an array.

Updates tailscale/corp#18320

Signed-off-by: Irbe Krumina <irbe@tailscale.com>

cmd/tailscale/cli/configure-kube_test.go

@@ -48,6 +48,31 @@ contexts:
 current-context: foo.tail-scale.ts.net
 kind: Config
 users:
+- name: tailscale-auth
+  user:
+    token: unused`,
+		},
+		{
+			name: "all configs, clusters, users have been deleted",
+			in: `apiVersion: v1
+clusters: null
+contexts: null
+kind: Config
+current-context: some-non-existent-cluster
+users: null`,
+			want: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://foo.tail-scale.ts.net
+  name: foo.tail-scale.ts.net
+contexts:
+- context:
+    cluster: foo.tail-scale.ts.net
+    user: tailscale-auth
+  name: foo.tail-scale.ts.net
+current-context: foo.tail-scale.ts.net
+kind: Config
+users:
 - name: tailscale-auth
   user:
     token: unused`,