Just played a team match.
I launched Null Matrix five times in a group of small minions (air scrats, crossbow dudes, legionnaires and puffs).
Two times the targeted minions had shield. The shield was not removed and they did not take any damage (maybe shield is not considered a positive effect and that's okay)
Three times, I targeted the same group of small minions. They had no positive effects and took zero damage.