input: Write a python function to join two lists.
  assert join_lists([1, 2, 3], [4, 5, 6]) == [1, 2, 3, 4, 5, 6]
  assert join_lists([], [4, 5, 6]) == [4, 5, 6]
  assert join_lists([1, 2, 3], []) == [1, 2, 3]
  assert join_lists([], []) == []
output: def join_lists(list1, list2):
      return list1 + list2