fix mypy

python/mlc_llm/serve/engine_base.py

@@ -667,7 +667,7 @@ def terminate(self):
         if hasattr(self, "_background_stream_back_loop_thread"):
             self._background_stream_back_loop_thread.join()
 
-    def _debug_call_func_on_all_worker(self, func_name: str, func_args: Optional[str]) -> None:
+    def _debug_call_func_on_all_worker(self, func_name: str, func_args: Optional[str] = None) -> None:
         """Call the given global function on all workers. Only for debug purpose."""
         self._ffi["debug_call_func_on_all_worker"](func_name, func_args)