caos.tree.transformer: Prevent spurious generation of Nones by path merger
caos.tree.transformer: Prevent spurious generation of Nones by path merger

Path merger was taking bad decisions on what to return in some edge
cases involving link property paths.  The result was that spurious None
values were generated where a valid query graph node was expected, thus,
crashing the transformer.

The root cause is dubious return logic in (add|intersect)_sets()

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb