Support pulling requestheader CA from extension-apiserver-authentication ConfigMap without client CA

This commit prevents extension API server from erroring out during bootstrap when the core
API server doesn't support certificate based authentication for it's clients i.e. client-ca isn't
present in extension-apiserver-authentication ConfigMap in kube-system.

This can happen in cluster setups where core API server uses Webhook token authentication.

Fixes: https://github.com/kubernetes/kubernetes/issues/65724