Check that the list size remains unchanged with an incorrect index

Add to the second parameterized test a check that the list size does not change, even if you try to get an element by an invalid index.

@ParameterizedTest
@ValueSource(ints = {-1, 10})
void shouldReturnEmptyWhenIndexIsIncorrect(int index) {
    Node<Integer> listNode = new ListNode<>(1);

    var result = listNode.get(index);

    assertTrue(result.isEmpty());
    assertEquals(1, listNode.size());
}