input: Write a function to implement the `IntList` class described in the given instruction. The function should initialize an empty list and provide methods to add values and find the minimum value of the list. If the list is empty, the `find_min` method 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
  
  # Test cases
  list1 = IntList()
  list1.add_value(5)
  list1.add_value(3)
  list1.add_value(8)
  assert list1.find_min() == 3
  
  list2 = IntList()
  list2.add_value(10)
  list2.add_value(2)
  assert list2.find_min() == 2
  
  list3 = IntList()
  list3.add_value(7)
  list3.add_value(7)
  assert list3.find_min() == 7
  
  list4 = IntList()
  assert list4.find_min() == None
output: 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