input: Write a function to swap the values of two variables. The function should take two parameters, swap them, and then return the swapped values. If no variables are provided, the function should return None.
  assert swap_values(5, 10)==(10, 5)
  assert swap_values(10, -3)==(-3, 10)
  assert swap_values('a', 'b')==('b', 'a')
  assert swap_values(True, False)==(False, True)
  assert swap_values(None, None)==None
output: def swap_values(a=None, b=None):
      if a is None or b is None:
          return None
      return b, a