2-2-4-linked-list, add tests accept the broken code

[obaibula]

The broken code:

private void addAsHead(Node<T> newNode) {
        newNode.next = head;
        head = newNode;
}

Please consider the following example:

LinkedList<Integer> list = new LinkedList<>();
        list.add(0, 100);
        list.add(144);

This code will result in a NullPointerException if we define the add method as follows:

@Override
    public void add(T element) {
        requireNonNull(element);
        Node<T> newNode = new Node<>(element);

        if(head == null){
            head = tail = newNode;
        } else {
            tail.next = newNode;
            tail = tail.next;
        }
        size++;
    }

It's necessary to reassign tail, as shown in the "completed" branch.