Fix meet_2 unwrapping a None when a source focus holds an empty node

[imlvts]

meet_2 guards both sources with try_as_tagged, then on the Identity arm unwraps into_option on the same value. The two disagree: try_as_tagged answers Some for a BorrowedRc whatever it holds, while into_option answers None when that node is empty. So a source whose focus holds an empty node passes the guard and panics on the unwrap. An empty node is what create_path leaves behind, so a read zipper rooted at a dangling path is enough:

a.create_path(&[0]);
wz.meet_2(&a.read_zipper_at_path(&[0]), &b.read_zipper_at_path(&[0]));

The mask on the Identity arm says which operand the intersection equals, so an empty operand means an empty intersection: graft nothing and report None. That is what the model specifies for meet_2 when either source node is empty.

Regression test: write_zipper_meet_2_with_an_empty_source_node, three shapes for the other operand, both operand orders. Fails before this change with an Option::unwrap() panic.