Server: don't add empty queues to persistent data

as revert breaks then, as it doesn't remove empty ones

dhfs-parent/server/src/main/java/com/usatiuk/dhfs/objects/jkleppmanntree/serializers/JKleppmannTreePersistentDataProtoSerializer.java

@@ -62,6 +62,7 @@ public class JKleppmannTreePersistentDataProtoSerializer implements ProtoSeriali
                 .setTreeName(object.getTreeName())
                 .setClock(object.getClock().peekTimestamp());
         for (var q : object.getQueues().entrySet()) {
+            if (q.getValue().isEmpty()) continue;
             var qb = builder.addQueuesBuilder();
             qb.setNode(q.getKey().toString());
             for (var e : q.getValue().entrySet()) {