Symfony2 Doctrine Expr ‘IS NOT NULL’

You can use isNotNull:

'query_builder' => function ($er){
    $qb = $er->createQueryBuilder('p');
    $qb
        ->where($qb->expr()->andx(
            $qb->expr()->in('p', '?1'),
            $qb->expr()->isNotNull('p.location')
        ))
        ->setParameter(1, $this->totalScope);

    return $qb;
},