input: Write a function to swap the values of two variables. If both variables are of the same type and can be swapped, return the swapped values. If either of the variables is None, return None for both.
  assert swap_variables(5, 10) == (10, 5)
  assert swap_variables(None, 10) == (None, None)
  assert swap_variables(5, None) == (None, None)
  assert swap_variables(None, None) == (None, None)
  assert swap_variables('a', 'b') == ('b', 'a')
output: def swap_variables(a, b):
      if a is None or b is None:
          return None, None
      return b, a