chore: initialize features variable

Author: kszucs

src/datasets/arrow_writer.py

@@ -430,6 +430,7 @@ def close(self):
 
     def _build_schema(self, inferred_schema: pa.Schema):
         schema = self.schema
+        features = self._features
         inferred_features = Features.from_arrow_schema(inferred_schema)
         if self._features is not None:
             if self.update_features:  # keep original features it they match, or update them