input: Write a function to print the absolute difference between two given numbers. If the first number is empty then return 0.
  assert absolute_difference(9, 5)==4
  assert absolute_difference(-3, 2)==5
  assert absolute_difference(0, 0)==0
  assert absolute_difference(-7, -2)==5
  assert absolute_difference(8, -4)==12
output: def absolute_difference(a, b):
      if a is None:
          return 0
      return abs(a - b)