check rerank document is not empty (#1740)

Co-authored-by: jyong <jyong@dify.ai>

api/core/model_providers/models/reranking/cohere_reranking.py

@@ -24,7 +24,10 @@ class CohereReranking(BaseReranking):
 
         super().__init__(model_provider, client, name)
 
-    def rerank(self, query: str, documents: List[Document], score_threshold: Optional[float], top_k: Optional[int]) -> Optional[List[Document]]:
+    def rerank(self, query: str, documents: List[Document], score_threshold: Optional[float], top_k: Optional[int]) -> \
+            Optional[List[Document]]:
+        if not documents:
+            return []
         docs = []
         doc_id = []
         unique_documents = []

api/core/model_providers/models/reranking/xinference_reranking.py

@@ -21,6 +21,8 @@ class XinferenceReranking(BaseReranking):
         super().__init__(model_provider, client, name)
 
     def rerank(self, query: str, documents: List[Document], score_threshold: Optional[float], top_k: Optional[int]) -> Optional[List[Document]]:
+        if not documents:
+            return []
         docs = []
         doc_id = []
         unique_documents = []