input: Write a function to swap the values of two variables, x and y. The function should return the swapped values of x and y. If either x or y is not provided, the function should return the original values of x and y.
  assert swap_values(x=5, y=10) == (10, 5)
  assert swap_values(x=15, y=20) == (20, 15)
  assert swap_values(x=30) == (30, None)
  assert swap_values(y=40) == (None, 40)
  assert swap_values() == (None, None)
output: def swap_values(x=None, y=None):
      return (y, x)