Seems to be caused by c53e220aef. I didn't think our PBVH building would
ever create a node with no triangles, but I guess I was wrong, since I
checked and the PBVH is clearly being rebuilt after the trim operation,
so it seems there isn't anything else weird going on here.
Rather than trying to figure that out, I'll just add the empty check
back here. Eventually we should completely replace the algorithm that
creates the PBVH nodes anyway.
768c68f19b