input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. Return the results as a tuple containing the maximum element, minimum element, and mean element. If the string is empty, return (None, None, None).
  assert get_stats_for_string("abcdef")==('f', 'a', 'd')
  assert get_stats_for_string("zyxk")==('z', 'k', 'w')
  assert get_stats_for_string("")==(None, None, None)
  assert get_stats_for_string("12345")==('5', '1', '3.0')
  assert get_stats_for_string("aA")==('a', 'A', 'Z')
output: def get_stats_for_string(s):
      if not s:
          return (None, None, None)
      max_char = max(s)
      min_char = min(s)
      mean_char = sorted(s)[len(s)//2]
      return (max_char, min_char, mean_char)