This is similar solution, which works with non numerical colums/expressions:
/* @var $datasource QueryBuilder */
$datasource->addSelect('CASE WHEN xxx.yyy IS NULL THEN 1 ELSE 0 END as HIDDEN yyy_is_null');
$datasource->orderBy('yyy_is_null', 'ASC'); // always ASC
$datasource->addOrderBy('xxx.yyy','DESC'); //DESC or ASC