public Model addClientOptional(Model model, boolean applyWhenNoDefaultValue) {
        return new AddClientOptional(applyWhenNoDefaultValue).transform(this, model);
    }
Add the clientOptional trait to members that are effectively nullable because
they are part of a structure marked with the input trait or they aren't required
and don't have a default trait.

@param model Model to transform.
@param applyWhenNoDefaultValue Set to true to add clientOptional to members that target
                               shapes with no zero value (e.g., structure and union).
@return Returns the transformed model.
smithy-model/src/main/java/software/amazon/smithy/model/transform/ModelTransformer.java