input: Write a function to complete the Python class `Sum` so that it can calculate the sum of two integers. If the class receives an empty input, it should return `None`. Include the function header and some assert tests.
  assert Sum(2, 3).add() == 5
  assert Sum(0, 0).add() == 0
  assert Sum(-1, 1).add() == 0
  assert Sum(10, -5).add() == 5
  assert Sum(5, -7).add() == -2
output: class Sum:
      def __init__(self, a, b):
          self.a = a
          self.b = b
  
      def add(self):
          if self.a is None or self.b is None:
              return None
          return self.a + self.b