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

db.collection.aggregate([{$addFields:{values_clone:{$map:{input:"$values",as:"v",in:{$cond:{if:{$eq:["$$v",null]},then:"null",else:"$$v"}}}}}},{$sort:{values_clone:1}},{$project:{values_clone:0}}])