Fixes #20748 KnowledgeRetrievalNode return all external documents when reranker disabled even top-k configed (#20762)

api/core/rag/retrieval/dataset_retrieval.py

                     all_documents = self.calculate_keyword_score(query, all_documents, top_k)
                 elif index_type == "high_quality":
                     all_documents = self.calculate_vector_score(all_documents, top_k, score_threshold)
+                else:
+                    all_documents = all_documents[:top_k] if top_k else all_documents
 
         self._on_query(query, dataset_ids, app_id, user_from, user_id)