input: Write a function that implements the `IntList` class as described in the instruction. The class should have an initializer to create an empty list and a method to add a value to the list. Additionally, it should have a method to find and return the minimum value in the list.
  
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
      
      def find_min(self):
          return min(self.values)
  
  assert IntList().add_value(5).find_min()==5
  assert IntList().add_value(7).add_value(3).find_min()==3
  assert IntList().add_value(2).add_value(4).add_value(1).find_min()==1
  assert IntList().add_value(8).add_value(6).add_value(9).find_min()==6
  assert IntList().find_min()==None
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
  