Automatic merge from submit-queue (batch tested with PRs 44606, 46038)

Fix serialization of EnforceNodeAllocatable

EnforceNodeAllocatable being `nil` and `[]` are treated in different
ways by kubelet. Namely, `nil` is replaced with `[]string{"pods"}` by
the defaulting mechanism.

E.g. if you run kubelet in Docker-in-Docker environment
you may need to run it with the following options:
`--cgroups-per-qos=false --enforce-node-allocatable=`
(this corresponds to EnforceNodeAllocatable being empty array and not
null) If you then grab kubelet configuration via /configz and try to
reuse it for dynamic kubelet config, kubelet will think that
EnforceNodeAllocatable is null, failing to run in the
Docker-in-Docker environment.

Encountered this while updating Virtlet for Kubernetes 1.6
(the dev environment is based on kubeadm-dind-cluster)