input: Write a function to calculate the average of a list of positive integers and output the result.
  
  assert list_average([1, 5, 6, 7, 8])==5.4
  assert list_average([10, 20, 30, 40, 50])==30.0
  assert list_average([1, 1, 1, 1])==1.0
  assert list_average([5])==5.0
  assert list_average([])==None
output: def list_average(lst):
      if not lst:
          return None
      return sum(lst) / len(lst)