input: Write a function to complete the Python class below so that it can calculate the sum of two integers. The class should have an __init__ method to initialize the integers and an add method to return the sum of the two integers. If the given integers are empty then return 0.
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          return self.x + self.y
  
  assert Sum(1, 2).add() == 3
  assert Sum(0, 0).add() == 0
  assert Sum(-1, 1).add() == 0
  assert Sum(5, -5).add() == 0
  assert Sum(-3, -3).add() == -6
output: def __init__(self, x, y):
      self.x = x if x is not None else 0
      self.y = y if y is not None else 0
  
  def add(self):
      return self.x + self.y