Check that -blocknotify command is non-empty before executing

To make BlockNotifyCallback(...) (-blocknotify) consistent with:
* AlertNotify(...) (-alertnotify)
* AddToWallet(...) (-walletnotify)

src/init.cpp

@@ -547,10 +547,11 @@ static void BlockNotifyCallback(bool initialSync, const CBlockIndex *pBlockIndex
         return;
 
     std::string strCmd = gArgs.GetArg("-blocknotify", "");
-
+    if (!strCmd.empty()) {
         boost::replace_all(strCmd, "%s", pBlockIndex->GetBlockHash().GetHex());
         boost::thread t(runCommand, strCmd); // thread runs free
     }
+}
 
 static bool fHaveGenesis = false;
 static boost::mutex cs_GenesisWait;