Feature: Support set chats kbs to empty (#9038)

### What problem does this PR solve?
https://github.com/infiniflow/ragflow/issues/9034

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/apps/sdk/chat.py

@@ -166,7 +166,7 @@ def update(tenant_id, chat_id):
         kbs = KnowledgebaseService.get_by_ids(ids)
         embd_ids = [TenantLLMService.split_model_name_and_factory(kb.embd_id)[0] for kb in kbs]  # remove vendor suffix for comparison
         embd_count = list(set(embd_ids))
-        if len(embd_count) != 1:
+        if len(embd_count) > 1:
             return get_result(message='Datasets use different embedding models."', code=settings.RetCode.AUTHENTICATION_ERROR)
         req["kb_ids"] = ids
     llm = req.get("llm")