input: Write a function to calculate the maximum, minimum, and mean values of the elements in a given string. Return these values as a tuple in the order: maximum, minimum, and mean. If the string is empty, return (None, None, None).
  
  assert get_stats_for_string("abcdef") == ('f', 'a', 6.5)
  assert get_stats_for_string("12345") == ('5', '1', 3.0)
  assert get_stats_for_string("") == (None, None, None)
output: def get_stats_for_string(s):
      if not s:
          return (None, None, None)
      max_val = max(s)
      min_val = min(s)
      mean_val = sum(ord(c) for c in s) / len(s)
      return (max_val, min_val, mean_val)