input: Write a function to print the absolute difference between two given numbers. If the function is given no numbers, it should return None.
    
  assert absolute_difference(9, 5)==4
  assert absolute_difference(-9, 5)==14
  assert absolute_difference(0, 0)==0
  assert absolute_difference() is None
output: def absolute_difference(a=None, b=None):
      if a is None or b is None:
          return None
      return abs(a - b)