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([{$facet:{"null":[{$match:{$or:[{userId:{$exists:false}},{userId:null}]}},{$count:"count"}],"nonNull":[{$match:{$and:[{userId:{$exists:true}},{userId:{$ne:null}}]}},{$count:"count"}]}},{$project:{"null":{$ifNull:[{$arrayElemAt:["$null.count",0]},0]},"nonNull":{$ifNull:[{$arrayElemAt:["$nonNull.count",0]},0]}}}])