message: Fix consumer friendly messages removing from PT
  In case PTU comes with omitted consumer_friendly_messages param
  SDL should maintain current consumer_friendly_messages section
  in Local PT.
diff: @@ -1177,10 +1177,12 @@ bool SQLPTRepresentation::SaveConsumerFriendlyMessages(
  // the policy table. So it won't be changed/updated
  if (messages.messages.is_initialized()) {
  utils::dbms::SQLQuery query(db());
  + if (!messages.messages->empty()) {
  if (!query.Exec(sql_pt::kDeleteMessageString)) {
  LOG4CXX_WARN(logger_, "Incorrect delete from message.");
  return false;
  }
  + }
  if (query.Prepare(sql_pt::kUpdateVersion)) {
  query.Bind(0, messages.version);
  