Last line in populateModel below is cruft. The Resource has 'resource-adapter' children; it can't (and doesn't) also have a 'resource-adapter' field.

class ResourceAdaptersSubsystemAdd extends AbstractAddStepHandler {

    static final ResourceAdaptersSubsystemAdd INSTANCE = new ResourceAdaptersSubsystemAdd();

    protected void populateModel(ModelNode operation, ModelNode model) {
        model.setEmptyObject();
        model.get(RESOURCEADAPTER_NAME);
    }

This looks like cruft that survived from very early days before Resource was introduced and the entire model was on big DMR tree.

This is just a minor thing I noticed when analyzing undescribed or runtime-only attributes that end up in the DMR models in the management resource tree as part of my work on WFCORE-4183 and WFCORE-4185.