default String renderJavadoc(Generator gen, M model) {
        return gen.on(model.getJavadoc())
            .map(doc -> doc + Formatting.nl())
            .orElse("");
    }
Render the javadoc-part of the model with an extra new-line appended
afterwards. If no javadoc exists, an empty string is returned.

@param gen    the generator
@param model  the model
@return       the generated code
common-parent/codegen/src/main/java/com/speedment/common/codegen/internal/java/view/trait/HasJavadocView.java