code: private static ApiFuture<Void> transformToVoid(ApiFuture<Empty> future) {
      return ApiFutures.transform(
          future,
          new ApiFunction<Empty, Void>() {
            @Override
            public Void apply(Empty empty) {
              return null;
            }
          },
          MoreExecutors.directExecutor());
    }
docs: Helper method to transform ApiFuture<Empty> to ApiFuture<Void>