input: Write a function to swap the values of two variables. If the given variables are empty then return them unchanged.
  
  assert swap_variables(5, 10) == (10, 5)
  assert swap_variables(15, 20) == (20, 15)
  assert swap_variables(0, 0) == (0, 0)
  assert swap_variables("", "") == ("", "")
  assert swap_variables(None, None) == (None, None)
output: def swap_variables(a, b):
      if a is None or b is None:
          return a, b
      return b, a