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, minimum element, and the mean of the elements in the string. If the string is empty, return (None, None, None).
  assert get_stats_for_string("abcdef")==('f', 'a', 2.5)
  assert get_stats_for_string("hello")==('o', 'e', 10/5)
  assert get_stats_for_string("")==(None, None, None)
output: def get_stats_for_string(s):
      if not s:
          return (None, None, None)
      max_elem = max(s)
      min_elem = min(s)
      mean_elem = sum(ord(c) for c in s) / len(s)
      return (max_elem, min_elem, mean_elem)