[{"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:{$cond:{if:{$eq:["$FirstName",null]},then:"$LastName",else:{$concat:["$FirstName"," ","$LastName"]}}}}}])