input: Write a python function to implement a queue class with methods to check if the queue is empty, add an item, remove an item, and check the size of the queue.
  assert queue.is_empty() == True
  queue.enqueue(1)
  assert queue.is_empty() == False
  queue.enqueue(2)
  assert queue.size() == 2
  assert queue.dequeue() == 1
  assert queue.size() == 1
output: class Queue:
      def __init__(self):
          self.items = []
  
      def is_empty(self):
          return len(self.items) == 0
  
      def enqueue(self, item):
          self.items.append(item)
  
      def dequeue(self):
          if not self.is_empty():
              return self.items.pop(0)
          return None
  
      def size(self):
          return len(self.items)
  
  queue = Queue()
  assert queue.is_empty() == True
  queue.enqueue(1)
  assert queue.is_empty() == False
  queue.enqueue(2)
  assert queue.size() == 2
  assert queue.dequeue() == 1
  assert queue.size() == 1