Database
[{"_id":ObjectId("5a934e000102030405000001"),"test":{"123123":{"someField":null}}},{"_id":ObjectId("5a934e000102030405000002"),"test":{"323143":{"someField":"lalala"},"121434":{"someField":null}}},{"_id":ObjectId("5a934e000102030405000003"),"test":{"4238023":{"someField":"afafa"}}},]
Query
db.collection.aggregate([{"$project":{"arr":{"$objectToArray":"$$ROOT.test"}}},{"$match":{arr:{$elemMatch:{"v.someField":{$ne:null}}}}},{"$project":{"_id":1,"test":{$arrayToObject:"$arr"}}}])