input: Write a function to calculate the maximum, minimum, and mean number of elements in a given string. If the given string is empty, return None.
  assert get_stats_for_string("abcdef") == ('f', 'a', 99.0)
  assert get_stats_for_string("HelloWorld") == ('r', 'H', 103.5)
  assert get_stats_for_string("") == None
output: def get_stats_for_string(s):
      if not s:
          return None
      max_char = max(s)
      min_char = min(s)
      mean_val = sum(ord(c) for c in s) / len(s)
      return max_char, min_char, mean_val