Fix ``Portal.run_in_actor()`` returns ``None`` result.

``None`` was being used as the cached result flag and obviously breaks
on a ``None`` returned from the remote target task. This would cause an
infinite hang if user code ever called ``Portal.result()`` *before* the
nursery exit. The simple fix is to use the ``Channel.uid`` as the
initial "no-result-received-yet" flag.