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