[{"_id":1231333,"cars":{"type-123":{"attribute":"faster"},"type-1234":{"attribute":"slow"},"type-12345":{"attribute":""}}}]

db.collection.aggregate({"$set":{"cars":{"$objectToArray":"$cars"}}},{"$unwind":"$cars"},{"$match":{"cars.v.attribute":{"$ne":""}}},{"$group":{"_id":"$_id","cars":{"$push":"$cars"}}},{"$set":{"cars":{"$arrayToObject":"$cars"}}})