input: Write a function to create a data structure in Python that can store integer values and has methods to add a new integer and find the minimum integer in the stored values. If the list of values 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(3)
  int_list.add_value(10)
  int_list.add_value(1)
  int_list.add_value(2)
  assert int_list.find_min()==1
  int_list = IntList()
  assert int_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
  