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>
}

まず QueryBuilder API と既存の filter() の処理を見つけ、次にクエリ内で ToMany リレーションがどのように表現されているかを確認します。empty(SomeEntity_.otherEntities) 形式のクエリが関連する OtherEntity レコードを持たないエンティティを返し、適切なクエリテストでカバーされれば作業は完了です。