input: Write a function to swap the values of two variables in Python. The function should take two arguments, x and y, and return the swapped values. If either of the variables is not provided, the function should return the original values of x and y.
  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, 5) == (None, 5)
  assert swap_values(1, None) == (1, None)
output: def swap_values(x=None, y=None):
      if x is None or y is None:
          return x, y
      return y, x