input: Write a function to calculate the average of a list of positive integers and output the result as a formatted string. The function should return None if the input list is empty.
  assert calculate_average([1, 5, 6, 7, 8]) == "The average of the list of positive integers is 5.4"
  assert calculate_average([10, 20, 30]) == "The average of the list of positive integers is 20.0"
  assert calculate_average([]) == None
output: def calculate_average(lst):
      if not lst:
          return None
      avg = sum(lst) / len(lst)
      return f"The average of the list of positive integers is {avg}"