fix response type for ModelController DELETE

Author: hamishfagg

ninja_extra/controllers/model/endpoints.py

@@ -460,7 +460,7 @@ def _setup(model_controller_type: t.Type["ModelControllerBase"]) -> t.Callable:
             return route.delete(
                 working_path,
                 url_name=url_name,
-                response={status_code: str},
+                response={status_code: None},
                 description=description,
                 operation_id=operation_id,
                 summary=summary,