Cyclops' power is...a bit of a weird one. He projects concussive "blasts" from his "eyes". Now, as per the Marvel wiki, they seem to have retconned it into an extra-dimensional energy that exits through "apertures" that replaced his eyes or are in his eyes or some such shit.
Now, supposedly, when this energy hits him (as in Cyclops, or Havok, I suppose) it gets shunted back into the dimension it came from. It also breaks a few other laws of physics by not applying equal force onto Cyclops himself (unless he wants it to, somehow, ala his '97 skidding around the floor intro) which I assume is why his "most powerful 500 lbs per square inch blast" doesn't just snap his neck when he shoots it off.
Anyway, as I see it there are 2 possibilities : either the blast hits Null and behaves as if it hit Cyclops and either gets nullified or somehow shunted back into its original dimension, or, more likely, it just hits Null and floors him, because once it's "fired" it's already here in this dimension as a form of energy doing its thing. I don't really think Cyke's "awareness" plays a role in this case.
It'd be a different story if Null was touching him / had a nullification field that touched him. Then he wouldn't be able to shoot a blast at all.
I once came up with a MUCH better explanation for Cyclops' powers. It begins with a blurb at the end of one of the early X-Men comics (somewhere around issue 30-40), where it explains that his body absorbs solar energy and makes the beams.
My addition to that explains how the beam works. Cyclops' body absorbs solar energy (and ambient other energies), and manufactures unstable particles charged with that energy. These particles are stored in the less dense areas of his body, but can escape through his eyes. They exit under pressure, like a water hose.
When the particles come in contact with less dense materials, such as air, they draw energy from the molecules they're in contact with, preserving their momentum. The process isn't efficient, however, so there is a low frequency waste photon during each interaction (red light).
When the particles come in contact with materials too dense to draw energy from, they instead transfer their own energy to the subject, as kinetic energy. The mass of particles has an impact of their own, but the released energy can be titanic. Because the energy release happens at the point of impact, the particle beam does not apply much force on Cyclops himself, which is why firing his beam doesn't knock HIM flying. Within his own body, the particles release their energy constantly, but his body reabsorbs it and makes new particles.
He can force the flow of particles to go faster, but he can expend all his stored particles this way. This used to happen to him often in the old, OLD days.
The particles have a unique interaction with ruby quartz. On contacting ruby quarts, the particles rebound. Within his visor, they are bounced back toward his face, where they're reabsorbed. However, by opening the visor aperture in different configurations, he can cause a coherent beam with different effects, such as spinning the particles so they will bounce off surfaces, or narrowing or widening a coherent beam, like water from a hose.
All this explains how his power works in detail, and explains why the beam functions as it does.
The beam is red because of the waste photons.
He is immune to the beam because his body absorbs the particles. It's also why he's immune to Havok's cosmic blasts, and why he's so resistant to other kinds of energy blasts.
The force applied to him on firing is based only on the flow of particles, not the energy IN them, so he doesn't knock himself back.
The impact of the particles is far greater than the mass of the particles should allow, because of the energy within them.
He is able to direct the particles using his visor.
He is able to push more particles for greater force.
But... Marvel, in its infinite wisdom, decided that it makes more sense that his eyes are holes into another universe.
