Add Empty.{completed,immediate}Future()

yang.common.Empty is useful as a non-null no-semantics object. It would be useful if there were a place where we could get the equivalent of Futures.immediateVoidFuture().

Introduce Empty.immediateFuture(), which will fill this role. Since we are not taking sides, also add Empty.completedFuture(), which does the same for CompletionStages.