input: Write a function to calculate the maximum, minimum, and mean character within a given string. If the string is empty, return (None, None, None).
  
  def get_stats_for_string(string):
      if len(string) == 0:
          return (None, None, None)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(ord(char) for char in string) / len(string)
      return (max_element, min_element, mean_element)
  
  assert get_stats_for_string("abcdef")==('f', 'a', 100.0)
  assert get_stats_for_string("")==(None, None, None)
  assert get_stats_for_string("zxy")==('z', 'x', 121.33333333333333)
output: def get_stats_for_string(string):
      if len(string) == 0:
          return (None, None, None)
      max_element = max(string)
      min_element = min(string)
      mean_element = sum(ord(char) for char in string) / len(string)
      return (max_element, min_element, mean_element)