input: Write a python function to implement a stack with push and pop operations.
  assert stack.push(1) == None
  assert stack.push(2) == None
  assert stack.pop() == 2
  assert stack.pop() == 1
  assert stack.pop() == None
output: class Stack:
      def __init__(self):
          self.items = []
      
      def push(self, item):
          self.items.append(item)
      
      def pop(self):
          return self.items.pop() if self.items else None
  
  stack = Stack()