fix: typing on _fn_result_to_body was incorrect which was caused an e…

…xception with cython3 (#210)

src/dbus_fast/service.py

@@ -504,7 +504,9 @@ def _msg_body_to_args(msg: Message) -> List[Any]:
 
     @staticmethod
     def _fn_result_to_body(
-        result: List[Any], signature_tree: SignatureTree, replace_fds: bool = True
+        result: Optional[List[Any]],
+        signature_tree: SignatureTree,
+        replace_fds: bool = True,
     ) -> Tuple[List[Any], List[int]]:
         """The high level interfaces may return single values which may be
         wrapped in a list to be a message body. Also they may return fds