[{"values":[1,2]},{"values":[5,null]},{"values":[-5,null]},{"values":[null,null]}]

db.collection.aggregate([{$project:{values:{$map:{input:"$values",as:"d",in:{$cond:{if:{$eq:["$$d",null]},then:"null",else:"$$d"}}}}}},{"$sort":{"values":1}},{$project:{values:{$map:{input:"$values",as:"d",in:{$cond:{if:{$eq:["$$d","null"]},then:null,else:"$$d"}}}}}}])