[{"first":"Test","last":"Name"},{"full":"Test_Name 2","first":"Test","last":"Test Name 2"}]

db.collection.aggregate([{"$project":{"fullName":{$switch:{branches:[{case:{$or:[{$lt:["$full",null]},{$eq:["$full",null]}],},then:{"$concat":["$first","_","$last"]}}],default:"$full"}}}}])