input: Write a function to swap the values of two variables. The function should take two arguments, the first variable and the second variable. It should swap their values and return them in a tuple. If either variable is None, return None.
  assert swap_variables(5, 10)==(10, 5)
  assert swap_variables(None, 3)==None
  assert swap_variables(7, None)==None
  assert swap_variables('a', 'b')==('b', 'a')
  assert swap_variables([1, 2], [3, 4])==([3, 4], [1, 2])
output: def swap_variables(var1, var2):
      if var1 is None or var2 is None:
          return None
      return (var2, var1)