Fix: handle missing dataset_ids when creating chat assistant (#9324)

- Root cause: accessing req.get("dataset_ids") returns None when the key
is absent, causing KeyError.
- Fix: use req.get("dataset_ids", []) to default to empty list.

api/apps/sdk/chat.py

     res["llm"] = res.pop("llm_setting")
     res["llm"]["model_name"] = res.pop("llm_id")
     del res["kb_ids"]
-    res["dataset_ids"] = req["dataset_ids"]
+    res["dataset_ids"] = req.get("dataset_ids", [])
     res["avatar"] = res.pop("icon")
     return get_result(data=res)