input: Write a function to calculate the average of a list of positive integers and output the result. If the given list is empty then return None.
  assert average_of_list([1, 5, 6, 7, 8])==5.4
  assert average_of_list([10, 20, 30])==20.0
  assert average_of_list([1])==1.0
  assert average_of_list([])==None
output: def average_of_list(lst):
      if not lst:
          return None
      return sum(lst) / len(lst)