Add query filter that tests for empty ToMany

It would be nice if QueryBuilder would allow to test for emptiness without having to resort to using filter(). This would be especially useful with many-to-many relations if one could write something like this:

boxFor(SomeEntity::class)
    .query()
    .empty(SomeEntity_.otherEntities)
    .build()
    .find()

This would return all instances of SomeEntity that do not have any related OtherEntity if SomeEntity is defined like this:

class SomeEntity {
    lateinit var otherEntities: ToMany<OtherEntity>
}

Start by locating the QueryBuilder API and its existing filter() handling, then review how ToMany relations are represented in queries. The work is complete when an empty(SomeEntity_.otherEntities)-style query returns entities with no related OtherEntity records and is covered by appropriate query tests.