input: Write a function to complete the Python class `Sum` so that it can calculate the sum of two integers. The class should include a method `add` that returns the sum of the two integers. If the given input is empty then return 0.
  
  
  class Sum:
      def __init__(self, x, y):
          self.x = x
          self.y = y
          
      def add(self):
          if self.x is None or self.y is None:
              return 0
          return self.x + self.y
  
  assert Sum(3, 4).add() == 7
  assert Sum(5, 10).add() == 15
  assert Sum(None, 2).add() == 0
  assert Sum(3, None).add() == 0
  assert Sum(None, None).add() == 0
  
output: def add(self):
      if self.x is None or self.y is None:
          return 0
      return self.x + self.y