input: Write a function to swap the values of two variables. If the given variables are empty then return the original input. 
  assert swap_variables(5, 10) == (10, 5)
  assert swap_variables(10, 5) == (5, 10)
  assert swap_variables(0, -1) == (-1, 0)
  assert swap_variables(None, None) == (None, None)
output: def swap_variables(a, b):
      if a is None and b is None:
          return a, b
      return b, a