input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. If the string is empty, return (None, None, None).
  
  assert get_stats_for_string("abcdef") == ('f', 'a', (ord('a') + ord('b') + ord('c') + ord('d') + ord('e') + ord('f')) / 6)
  assert get_stats_for_string("1234567890") == ('9', '0', (ord('0') + ord('1') + ord('2') + ord('3') + ord('4') + ord('5') + ord('6') + ord('7') + ord('8') + ord('9')) / 10)
  assert get_stats_for_string("") == (None, None, None)
output: def get_stats_for_string(s):
      if not s:
          return (None, None, None)
      max_char = max(s)
      min_char = min(s)
      mean_value = sum(ord(c) for c in s) / len(s)
      return (max_char, min_char, mean_value)