input: Write a function to find the common elements in two lists. Return a list containing the common elements between the two input lists. If one of the lists is empty, return an empty list.
  assert common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert common_elements([], [21, 7, 13, 29, 31])==[]
  assert common_elements([3, 9, 15, 21, 27], [])==[]
  assert common_elements([], [])==[]
  assert common_elements([1, 2, 3], [3, 2, 1])==[1, 2, 3]
output: def common_elements(list1, list2):
      if not list1 or not list2:
          return []
      return list(set(list1) & set(list2))