input: Write a function to implement the `IntList` class, which stores integer values and provides methods to add values and find the minimum value. The function should return the minimum value from the list if it is not empty, otherwise it should return None.
  
  class IntList:
      def __init__(self):
          self.values = []
     
      def add_value(self, value):
          self.values.append(value)
      
      def find_min(self):
          if self.values:
              return min(self.values)
          else:
              return None
  
  assert IntList().find_min()==None
  assert IntList().add_value(5).find_min()==5
  assert IntList().add_value(10).add_value(3).add_value(7).find_min()==3
  assert IntList().add_value(-2).add_value(0).add_value(3).find_min()==-2
output: def __init__(self):
      self.values = []
  
  def add_value(self, value):
      self.values.append(value)
      return self
  
  def find_min(self):
      if self.values:
          return min(self.values)
      else:
          return None