Revert `RCell` changes -- disallow empty.
Revert `RCell` changes -- disallow empty.

reactors-io/reactors,storm-enroute/reactors,storm-enroute/reactors,reactors-io/reactors,storm-enroute/reactors,reactors-io/reactors,storm-enroute/reactors,reactors-io/reactors,storm-enroute/reactive-collections