input: Write a function to swap the values of two variables. The function should take two arguments and return the swapped values as a tuple. If either variable is not provided, the function should return the original values.
  assert swap_values(5, 10) == (10, 5)
  assert swap_values(10, 5) == (5, 10)
  assert swap_values(0, 0) == (0, 0)
  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=None, b=None):
      return (b, a)