input: Write a function to implement a data structure in Python that can store integer values, add new values, and find the minimum value among the stored integers. The data structure should return None when queried for the minimum value if no values have been added.
  
  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)
  
  assert IntList().find_min() == None
  int_list = IntList()
  int_list.add_value(5)
  int_list.add_value(3)
  int_list.add_value(7)
  assert int_list.find_min() == 3
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)
  