Always return the item in plugins unless there's an error

[sseago]

When returning the unmodified item in the migration common restore plugin in the case of cluster-scoped resources, return the item rather than nil. Returning nil results in velero errors like this: error preparing customresourcedefinitions.apiextensions.k8s.io/foos.example.foo.com: rpc error: code = Aborted desc = plugin panicked: runtime error: invalid memory address or nil pointer dereference

[dymurray]

@Danil-Grigorev FYI this was a regression introduced with #58 . When we want a skip a resource we need to return the unmodified object instead of nil.