input: Write a function to create a class that stores integer values and provides methods to add values and find the minimum value among them. 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
  
  assert IntList().find_min() == None
  int_list = IntList()
  int_list.add_value(10)
  int_list.add_value(5)
  int_list.add_value(3)
  assert int_list.find_min() == 3
  empty_list = IntList()
  assert empty_list.find_min() == None
output: 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