Automatic merge from submit-queue

Fix clientcmd for a non-host cluster client running in a pod. 

This is based on @errordeveloper's PR #32438. It fixes a case where default config is invalid and original config, i.e. `mergedConfig` is empty. It also adds a test for the case where default config is invalid and original config is neither invalid nor empty.

cc @errordeveloper @kubernetes/sig-cluster-federation @pwittrock @colhom