How to Fill Gaps Between Hardibacker and Drywall?
If you are going to paint, the seams between the drywall and the hardibacker will need to be mudded and taped like any other wallboard seam or the joint will be visible and ugly. it will probably be hyper-visible actually, if you use glossy paint (as you should in moist areas).

I think you'll be fine with standard mud/tape, it's not a wet location, just moist if it is indeed above the splash zone (if it's not above the splash zone, you should use an alternate surface treatment instead of simply hardibacker). A good prime and gloss paint should protect the drywall, mud joint, and hardibacker.