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', (97.0+98.0+99.0+100.0+101.0+102.0)/6)
  assert get_stats_for_string("zyxwv")==('z', 'v', (122+121+120+119+118)/5)
  assert get_stats_for_string("")==(None, None, None)
  assert get_stats_for_string("a")==('a', 'a', 97.0)
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)