input: Write a function to create a class named IntList which initializes an empty list to store integer values and has two methods: add_value to add an integer to the list and find_min to return the minimum value in the list. If the list is empty, find_min should return None.
  
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
      
      def find_min(self):
          return min(self.values) if self.values else None
  
  assert IntList().find_min()==None
  assert IntList().add_value(5).find_min()==5
  assert IntList().add_value(3).add_value(1).add_value(2).find_min()==1
output: class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
          return self
      
      def find_min(self):
          return min(self.values) if self.values else None