input: Write a function to find the common elements in two lists. If there are no common elements, return an empty list. If either list is empty, return an empty list as well.
  assert common_elements_finder([3, 9, 15, 21, 27], [21, 7, 13, 29, 31]) == [21]
  assert common_elements_finder([], [21, 7, 13, 29, 31]) == []
  assert common_elements_finder([3, 9, 15, 21, 27], []) == []
  assert common_elements_finder([1, 2, 3], [4, 5, 6]) == []
  assert common_elements_finder([1, 2, 3], [3, 2, 1]) == [1, 2, 3]
output: def common_elements_finder(list1, list2):
      if not list1 or not list2:
          return []
      return list(set(list1) & set(list2))