Automatic merge from submit-queue
return nil from NewClientConfig instead of empty struct
This is a go convention and fixes an nil pointer in kubelet when passing in bad command line options:
```
I0624 04:12:33.333246 25404 plugins.go:141] Loaded network plugin "kubenet"
E0624 04:12:33.333390 25404 runtime.go:58] Recovered from panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:52
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/runtime/runtime.go:40
/usr/local/go/src/runtime/asm_amd64.s:472
/usr/local/go/src/runtime/panic.go:443
/usr/local/go/src/runtime/panic.go:62
/usr/local/go/src/runtime/sigpanic_unix.go:24
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/client/clientset_generated/internalclientset/typed/core/unversioned/service.go:132
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/kubelet/kubelet.go:254
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/client/cache/listwatch.go:80
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/client/cache/reflector.go:262
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/client/cache/reflector.go:204
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/wait/wait.go:86
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/wait/wait.go:87
/go/src/k8s.io/kubernetes/_output/dockerized/go/src/k8s.io/kubernetes/pkg/util/wait/wait.go:49
```
cc @caesarxuchao @lavalamp
07d24e4557