input: Write a function to calculate the maximum, minimum and mean number of elements in a given string. The function should take a string as input and return a tuple containing the maximum, minimum, and mean values of the string elements. If the string is empty, the function should return `(None, None, None)`.
  
  assert get_stats_for_string("abcdef")==('f', 'a', 98.0)
  assert get_stats_for_string("12345")==('5', '1', 53.0)
  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)