Database
db={"myEntity":[{"creationDate":ISODate("2020-06-01"),"userId":"1"},{"creationDate":ISODate("2020-06-02"),"userId":"1"},{"creationDate":ISODate("2020-06-03"),"userId":"2"},{"creationDate":ISODate("2020-06-03"),"userId":"3"},{"creationDate":ISODate("2020-06-01"),"userId":null},{"creationDate":ISODate("2020-06-02"),"userId":null},{"creationDate":ISODate("2020-06-02")}]}
Query
db.myEntity.aggregate([{"$group":{"_id":"","null":{$sum:{$cond:[{$ne:[{"$type":"$userId"},"string"]},1,0]}},"nonNull":{"$sum":{$cond:[{$eq:[{"$type":"$userId"},"string"]},1,0]}}}}])