Automatic merge from submit-queue

kube-apiextensions-server: Fix potential SEGV with null delegate handler

**What this PR does / why we need it**:
In the kube-apiextensions-server there is a fallback value for `null` delegate to `http.NotFoundHandler()` in handling group and versions discovery, but no fallback for custom resources endpoint.
It leads to SEGV when running with `genericapiserver.EmptyDelegate`.