[{_id:ObjectId("619756f12c115f24df503c26"),uniqueid:"12345678",date1:"2021-11-02 20:04:50.253",date2:"2021-11-03 18:10:57.520",date3:"2021-11-08 07:08:00.000",date4:"2021-11-08 14:40:00.000",date5:"2021-11-08 08:34:00.000",cal_date1:"2021-11-06 18:10:57.520"}]

db.collection.aggregate({"$project":{"cal_days_between_date1_caldate1":{$switch:{branches:[{case:{$eq:["$date1",null]},then:"blank"},{case:{$eq:["$cal_date1","blank"]},then:"blank"}],default:{$dateDiff:{startDate:{$dateFromString:{dateString:"$date1"}},endDate:{$dateFromString:{dateString:"$cal_date1"}},unit:"day"}},}}}})