feat(core): Support simple DB query for sdk (#917)

Co-authored-by: chengfangyin2 <chengfangyin3@jd.com>

dbgpt/core/awel/task/task_impl.py

@@ -53,7 +53,7 @@ class SimpleTaskOutput(TaskOutput[T], Generic[T]):
 
     @property
     def is_empty(self) -> bool:
-        return not self._data
+        return self._data is None
 
     async def _apply_func(self, func) -> Any:
         if asyncio.iscoroutinefunction(func):