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>
}

首先定位 QueryBuilder API 及其现有的 filter() 处理，然后检查 ToMany 关系在查询中的表示方式。当 empty(SomeEntity_.otherEntities) 风格的查询返回没有相关 OtherEntity 记录的实体，并且有适当的查询测试覆盖时，工作即告完成。