Fix 'kube apply' panic on nil items

Bug 1146024 - https://bugzilla.redhat.com/show_bug.cgi?id=1146024
I guess it's just a performance improvement. The same pattern can be seen at:
- https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/runtime/scheme.go#L127
- https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/runtime/embedded.go#L31
- https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/util/time.go#L56