[{"FirstName":null,"LastName":"natra","Gender":null,"IsActive":true,},{"FirstName":"abiy","LastName":"jagema","Gender":"Female","IsActive":true,},]

db.collection.aggregate([{$match:{IsActive:true}},{$project:{Gender:1,IsActive:1,FullName:{$concat:[{$ifNull:["$FirstName",""]}," ",{$ifNull:["$LastName",""]}]}}}])