input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. The function should return a tuple containing the maximum element, the minimum element, and the mean value of the elements in the string.
  assert get_stats_for_string("abcdef") == ('f', 'a', sum("abcdef") / 6)
  assert get_stats_for_string("123456") == ('6', '1', sum("123456") / 6)
  assert get_stats_for_string("hello") == ('o', 'e', sum("hello") / 5)
  assert get_stats_for_string("world") == ('w', 'd', sum("world") / 5)
  assert get_stats_for_string("") == (None, None, None)
output: def get_stats_for_string(s):
      if not s:
          return (None, None, None)
      return (max(s), min(s), sum(map(ord, s)) / len(s))