default String renderValue(Generator gen, M model) {
        return ifelse(model.getValue(), 
			v -> " = " + gen.on(v).orElse(""), 
            ""
        );
    }
Render the value of the model if it exists, else an empty string.

@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/HasValueView.java