Avoid consistency problem when has no more data (#1716)

be/src/runtime/result_queue_mgr.cpp

@@ -47,6 +47,10 @@ Status ResultQueueMgr::fetch_result(const TUniqueId& fragment_instance_id, std::
         // sentinel nullptr indicates scan end
         if (*result == nullptr) {
             *eos = true;
+            // put sentinel for consistency, avoid repeated invoking fetch result when hava no rowbatch
+            if (queue != nullptr) {
+                queue->blocking_put(nullptr);
+            }
         } else {
             *eos = false;
         }