How to specify null value as filter in a Doctrine query?

If you are using Symfony 2 and above, you can use this code:

->where($qb->expr()->isNull('a.vertical_id'));

Reference:
http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/query-builder.html#the-expr-class

I use doctrine with symfony, and this is how I do:

where('a.vertical_id is NULL');