Fix python3.6 compatibility issue with dataclasses

Author: abn

src/betterproto/__init__.py

@@ -666,7 +666,8 @@ def _cls_for(cls, field: dataclasses.Field, index: int = 0) -> Type:
         """Get the message class for a field from the type hints."""
         field_cls = cls._type_hint(field.name)
         if hasattr(field_cls, "__args__") and index >= 0:
-            field_cls = field_cls.__args__[index]
+            if field_cls.__args__ is not None:
+                field_cls = field_cls.__args__[index]
         return field_cls
 
     def _get_field_default(self, field_name):