go to your mechanic and ask for a mechanical oil pressure check up, it should give you a true reading. if its reading fine on the mechanical gauge, get your sendor unit changed( it costs around 200 from nissan).
if its reading like that on the mechanical gauge as well, you are in serious trouble. it could be the oil pump or bearing.......
personally i think its your gauge, coz if its really zero oil pressure, you would start to hear some loud noise from your engine..., and its just a matter of mins before the motor is gone.