[fix](stmt-forward) fix forward null packet (#21979)

fe/fe-core/src/main/java/org/apache/doris/service/FrontendServiceImpl.java

@@ -908,6 +908,8 @@ public class FrontendServiceImpl implements FrontendService.Iface {
         if (params.isSyncJournalOnly()) {
             final TMasterOpResult result = new TMasterOpResult();
             result.setMaxJournalId(Env.getCurrentEnv().getMaxJournalId());
+            // just make the protocol happy
+            result.setPacket("".getBytes());
             return result;
         }