input: Write a function to implement the IntList class that stores integer values and provides methods to add values and find the minimum value among them. The function should return the minimum value after adding certain values to the list.
  assert IntList().add_value(3).add_value(1).add_value(2).find_min()==1
  assert IntList().add_value(5).add_value(-2).add_value(10).find_min()==-2
  assert IntList().add_value(0).add_value(0).add_value(0).find_min()==0
  assert IntList().add_value(9).add_value(7).add_value(8).find_min()==7
  assert IntList().find_min() is None
output: class IntList:
      def __init__(self):
          self.values = []
      
      def add_value(self, value):
          self.values.append(value)
          return self
      
      def find_min(self):
          if not self.values:
              return None
          return min(self.values)