input: Write a function to swap the values of two variables. If both variables are provided, return the swapped values. If one or both variables are missing, return the original input.
  assert swap_values(5, 10) == (10, 5)
  assert swap_values(None, 10) == (None, 10)
  assert swap_values(5, None) == (5, None)
  assert swap_values(None, None) == (None, None)
output: def swap_values(a, b):
      if a is not None and b is not None:
          return b, a
      return a, b