Fix missing extensions from protobuf

Author: yolagil

src/a2a/utils/proto_utils.py

@@ -579,7 +579,7 @@ def message(cls, message: a2a_pb2.Message) -> types.Message:
             task_id=message.task_id or None,
             role=cls.role(message.role),
             metadata=cls.metadata(message.metadata),
-            extensions=list(message.extensions) or None,
+            extensions=list(message.extensions) if message.extensions else None,
         )
 
     @classmethod