message: Return error for string type index
diff: @@ -108,6 +108,9 @@ private:
  }
  auto type = iter->get_type().get_type();
  if (IndexKeyUtils::toValueType(type) == Value::Type::STRING) {
  + if (!iter->get_type().__isset.type_length) {
  + return Status::Error("String property index has not set prefix length.");
  + }
  auto len = *iter->get_type().get_type_length();
  prefix.append(IndexKeyUtils::encodeValue(col.get_begin_value(), len));
  } else {
  