fix: map field edge-case

This change ensures a parent is a nested type when checking if a field is a map.

src/betterproto/plugin/models.py

@@ -339,6 +339,9 @@ def is_map(
 ) -> bool:
     """True if proto_field_obj is a map, otherwise False."""
     if proto_field_obj.type == FieldDescriptorProtoType.TYPE_MESSAGE:
+        if not hasattr(parent_message, "nested_type"):
+            return False
+
         # This might be a map...
         message_type = proto_field_obj.type_name.split(".").pop().lower()
         map_entry = f"{proto_field_obj.name.replace('_', '').lower()}entry"