Yeah it is too strict.
If you use default then you can do without strict variable for those sections.
Or you need to actually set the value even empty.
E.g. use defaults in config to set
headerImage: nil
Or maybe ''. Then it is set as a variable but then default should see it as empty and replace it.
Do you mean slugify not slugize ? That sounds like strict filter is doing its job to alert you.
See bottom of string filters list