input: Write a python function to implement a stack with methods to push, pop, check size, and check if it is empty.
  assert stack.is_empty() == True
  stack.push(1)
  assert stack.is_empty() == False
  assert stack.size() == 1
  stack.push(2)
  assert stack.size() == 2
  assert stack.pop() == 2
  assert stack.size() == 1
  assert stack.pop() == 1
  assert stack.is_empty() == True
output: class Stack:
      def __init__(self):
          self.items = []
  
      def push(self, item):
          self.items.append(item)
  
      def pop(self):
          return self.items.pop() if not self.is_empty() else None
  
      def size(self):
          return len(self.items)
  
      def is_empty(self):
          return len(self.items) == 0
  
  stack = Stack()