input: Write a function to find the common elements between two lists and return them as a new list. If the lists are empty, return an empty list.
  assert common_elements([1, 2, 3, 4, 8], [2, 6, 8, 10])==[2, 8]
  assert common_elements([1, 2, 3], [])==[]
  assert common_elements([], [2, 6, 8])==[]
  assert common_elements([1, 3, 4, 8], [2, 3, 4, 8, 10])==[3, 4, 8]
  assert common_elements([1, 2, 3, 4], [5, 6, 7, 8])==[]
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))