Fix join_into dropping the source when the destination node is empty

[imlvts]

Joining a non-empty source into an empty destination map reported Identity and wrote nothing; the same join into a destination holding one unrelated key worked. try_as_tagged answers Some for a BorrowedRc whatever it holds, so an empty destination node took the pjoin_dyn path and came back Identity(SELF_IDENT): "the result is what you already have", which for an empty destination is nothing.

The function already draws the distinction between "no node" and "an empty node" for the source, with an explicit node_is_empty check in the branch above; it did not draw it for the destination. Now it does, and an empty node takes the same path as a missing one, because the union of nothing with the source is the source. The model agrees: join_into with an empty self node joins to the source and reports Element.

Regression test: write_zipper_join_into_empty_destination, into an empty map, into a dangling path, and into a non-empty map. Fails before this change with the data missing.