alter way of alerting info of empty task (#2094)

### What problem does this PR solve?

#2043

### Type of change

- [x] Refactoring

rag/svr/task_executor.py

@@ -118,7 +118,10 @@ def collect():
         cron_logger.info("Task {} has been canceled.".format(msg["id"]))
         return pd.DataFrame()
     tasks = TaskService.get_tasks(msg["id"])
-    assert tasks, "{} empty task!".format(msg["id"])
+    if not tasks:
+        cron_logger.warn("{} empty task!".format(msg["id"]))
+        return []
+
     tasks = pd.DataFrame(tasks)
     if msg.get("type", "") == "raptor":
         tasks["task_type"] = "raptor"