input: Write a function to calculate the absolute difference between two given numbers. If either of the numbers is not provided, return 0.
  
  Example 1:
  Input: 9, 5
  Output:
  
  def absolute_difference(A, B): 
      if A is None or B is None:
          return 0
      return abs(A - B)
    
  # Driver Code 
  A = 9
  B = 5
  print(absolute_difference(A, B))
  
  Example 2:
  Input: 7, None
  Output:
  
  def absolute_difference(A, B): 
      if A is None or B is None:
          return 0
      return abs(A - B)
    
  # Driver Code 
  A = 7
  B = None
  print(absolute_difference(A, B))
  
  
  Assertions:
  
  assert absolute_difference(9, 5) == 4
  assert absolute_difference(7, None) == 0
  assert absolute_difference(None, 3) == 0
  
output: def absolute_difference(A, B): 
      if A is None or B is None:
          return 0
      return abs(A - B)