input: Write a function to generate a random number between the given input numbers. If the input range is invalid (e.g., start number is greater than the stop number), return None. The function should return the random number if the range is valid.
  def random_num(start, stop):
      if start > stop:
          return None
      return random.randint(start, stop)
  
  assert random_num(0, 10) is not None
  assert random_num(10, 0) is None
  assert random_num(5, 5) == 5
output: def random_num(start, stop):
      if start > stop:
          return None
      return random.randint(start, stop)