We used to set empty T_DATA nodes to delivered as soon as we encountered
them during unpending. While it seemed like a good idea to not spend
additional work with nodes which do not contain anything, the tree
integrity assertions as well as set_deliver() implicitly assume that

- a delivered node cannot follow an undelivered node unter a nexus and
- once the last node under a nexus is delivered, all of the nexus is
  delivered

Rather than sacrificing the sensible principle that a tree always is to
be delivered top to bottom, left to right, weakening our assertions and
possibly introducing new bugs when handling the resulting special cases,
we go for the simple, clean option and do away with a special case which
will not contribute much to overall performance anyway.