check rerank document is not empty (#1740)

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

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

 
         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 = []
                 docs.append(document.page_content)
                 unique_documents.append(document)
         documents = unique_documents
-        
+
         results = self.client.rerank(query=query, documents=docs, model=self.name, top_n=top_k)
         rerank_documents = []
 

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

         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 = []