message: Avoid callback when -blocknotify is empty
diff: @@ -1669,20 +1669,15 @@ bool AppInitMain(const util::Ref& context, NodeContext& node, interfaces::BlockA
  }
  #if HAVE_SYSTEM
  - if (args.IsArgSet("-blocknotify")) {
  const std::string block_notify = args.GetArg("-blocknotify", "");
  - const auto BlockNotifyCallback = [block_notify](SynchronizationState sync_state, const CBlockIndex* pBlockIndex) {
  - if (sync_state != SynchronizationState::POST_INIT || !pBlockIndex)
  - return;
  -
  - std::string strCmd = block_notify;
  - if (!strCmd.empty()) {
  - boost::replace_all(strCmd, "%s", pBlockIndex->GetBlockHash().GetHex());
  - std::thread t(runCommand, strCmd);
  + if (!block_notify.empty()) {
  + uiInterface.NotifyBlockTip_connect([block_notify](SynchronizationState sync_state, const CBlockIndex* pBlockIndex) {
  + if (sync_state != SynchronizationState::POST_INIT || !pBlockIndex) return;
  + std::string command = block_notify;
  + boost::replace_all(command, "%s", pBlockIndex->GetBlockHash().GetHex());
  + std::thread t(runCommand, command);
  t.detach(); // thread runs free
  - }
  - };
  - uiInterface.NotifyBlockTip_connect(BlockNotifyCallback);
  + });
  }
  #endif
  