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. If the string is empty, the function should return (None, None, None).
  
  assert get_stats_for_string("abcdef")==('f', 'a', 102.5)
  assert get_stats_for_string("12345")==('5', '1', 51.0)
  assert get_stats_for_string("")==(None, None, None)
output: def get_stats_for_string(s):
      if not s:
          return (None, None, None)
      elements = list(s)
      max_element = max(elements)
      min_element = min(elements)
      mean_value = sum(ord(c) for c in elements) / len(elements)
      return (max_element, min_element, mean_value)