There are plenty of reasons to believe that the Nulls would be capable of detecting cloaked enemies. The Republic Commando series outlines what the Null's Beskar'gam was capable of in a few passages, such as "Good things, helmets: nobody could hear what was being said on the comlink outside of the buy'ce unless you let them. Ordo was using all his state-of-the-art armor sensors to check for metal fatigue, leaks, and other mechanical faults. Skirata noted the readouts being relayed to his spanking-new HUD display" and "Ordo activated his helmet's infrared filter, adjusted it to its most sensitive setting, and cast around on a square search of a twenty-meter grid . . . Ordo switched from infrared to the penetrating sensor. It was like checking in mail slots, a tedious progression from one hole to the next."
And those are just excerpts from like two chapters of True Colors, I'm pretty sure I recall many other instances of high-tech thermal imaging or other types of sensors. There's no hiding from a Null. In lore, at least. Their clone armor was incredibly high-tech as well, with them making several upgrades to it throughout the course of the war- so I don't think it's too much of a stretch for their armor to be capable of the same thing the RC Armor is capable of (especially since the Nulls could take their pick of any Republic Technology they wanted seeing as they could get into anything and everything).
+1, allow it overall and not just when wearing Beskar'gam. I think the Null-Class ARCs being the only members of SOBDE who cannot see cloaked individuals has always been a bit annoying and dumb. Just round it out that all of SOBDE can see cloakers. As someone who owns the Guild Spy whitelist I can tell you that even with 10 people online who can see cloakers it's still possible to sneak around, you just actually have to try to be stealthy, which is much more fun and rewarding anyways.