The effect of Sudden Light is stun. If you are immune to stun for any reason, you should be immune to it.
RMU notes that if you are using nightvision (you have it and you are currently operating in dark conditions), you are more sensitive to blinding flashes of light. It would not be unreasonable to rule that a nightvision-using person/creature suffers some visual impairment even if immune to stun. Maybe for half the normal number of rounds you suffer -25 but no other effects of stun? Again, a GM judgment call, but it should be a much reduced effect, and should not stack with the normal effect if you are not immune to stun (in that case RMU gives a -25 penalty on the RR against Sudden Light as the only effect, which would result in a longer duration).
Creatures that are immune to stun and also don't use normal vision (e.g. skeletons) would be immune to both effects.