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>
(cherry picked from commit 56f7a09931e5c8d538913ffde570b7d6e0ae753e)
10ce099e60