input: Write a function to find the common elements in two lists. The function should return a list of common elements. If one of the lists is empty, it should return an empty list.
  assert find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert find_common_elements([1, 2, 3], [4, 5, 6])==[]
  assert find_common_elements([], [1, 2, 3])==[]
  assert find_common_elements([1, 2, 3], [])==[]
  assert find_common_elements([1, 2, 3], [3, 2, 1])==[1, 2, 3]
output: def find_common_elements(list1, list2):
      if not list1 or not list2:
          return []
      return list(set(list1) & set(list2))