fix(mypy): fixing mypy tests

src/lerobot/configs/policies.py

@@ -153,6 +153,8 @@ class PreTrainedConfig(draccus.ChoiceRegistry, HubMixin, abc.ABC):  # type: igno
 
     @property
     def audio_features(self) -> dict[str, PolicyFeature]:
+        if not self.input_features:
+            return {}
         return {key: ft for key, ft in self.input_features.items() if ft.type is FeatureType.AUDIO}
 
     @property