Why don't you have any coding standards? If you're working with agents in particular, catching and enforcing this stuff should be easier than ever.
Everyone has to deal with other code that might not even be from the same org. The "check it in CI" answer isn't an excuse either. You're bolting on so much extra crap that way.
I just don't see why nullability is a problem in the first place.
Most engineers do not have the ability to impose rules by fiat on all of their coworkers. It seems like you're misunderstanding the nature of working on a codebase as an IC when other developers contribute to it. If all of my coworkers don't want a lint rule I propose, I don't get to add it. If all of my other coworkers want to write code in a certain way and approve each other's MRs with code written in that way, I don't get to veto it.