fix issue: query is none when doing retrieval (#10129)

api/core/rag/datasource/retrieval_service.py

         reranking_mode: Optional[str] = "reranking_model",
         weights: Optional[dict] = None,
     ):
+        if not query:
+            return []
         dataset = db.session.query(Dataset).filter(Dataset.id == dataset_id).first()
         if not dataset:
             return []