input: Write a python function to implement a stack using linked list and include push and pop operations.
  assert stack.pop() == None
  stack.push(10)
  stack.push(20)
  assert stack.pop() == 20
  assert stack.pop() == 10
  assert stack.pop() == None
output: class Node:
      def __init__(self, value):
          self.value = value
          self.next = None
  
  class Stack:
      def __init__(self):
          self.top = None
  
      def push(self, value):
          new_node = Node(value)
          new_node.next = self.top
          self.top = new_node
  
      def pop(self):
          if self.top is None:
              return None
          popped_value = self.top.value
          self.top = self.top.next
          return popped_value
  
  stack = Stack()