input: Write a function to implement the `IntList` class for storing integer values, including methods to add a value and find the minimum value in the list. The function should handle the addition of values and retrieval of the minimum value correctly. If the list is empty, the find_min method should return None.
  assert IntList().find_min() == None
  int_list = IntList()
  int_list.add_value(3)
  int_list.add_value(5)
  int_list.add_value(1)
  int_list.add_value(2)
  assert int_list.find_min() == 1
output: class IntList:
      def __init__(self):
          self.values = []
  
      def add_value(self, value):
          self.values.append(value)
  
      def find_min(self):
          if not self.values:
              return None
          return min(self.values)