input: Write a function to print the absolute difference between two given numbers. If either of the numbers is negative then return None.
  assert absolute_difference(9, 5)==4
  assert absolute_difference(9, -5)==None
  assert absolute_difference(-9, 5)==None
  assert absolute_difference(-9, -5)==None
  assert absolute_difference(9, 9)==0
output: def absolute_difference(a, b):
      if a < 0 or b < 0:
          return None
      return abs(a - b)