docs: def validate_appname_or_none(option, value):
      """Validate the appname option."""
      if value is None:
          return value
      validate_string(option, value)
      # We need length in bytes, so encode utf8 first.
      if len(value.encode('utf-8')) > 128:
          raise ValueError("%s must be <= 128 bytes" % (option,))
      return value