Given the head of a singly linked list, delete the last node and return the head of the resulting list. If the list is empty or has just one node, the result is an empty list. Unlike deleting the first node, this one still requires a walk from head — a singly linked list keeps no pointer to its own tail, so the second-to-last node has to be found by checking one node ahead at each step (curr.next.next == null). Once found, dropping the last node is a single pointer write.