Empty sump is just an open-top tank. With that, evaporation and debris (dust) are your only real concerns. In terms of an overnight timeline; no big deal at all.
Rock only would add a heating concern. If the sump got too cold for too long, some of the bacteria might die off or go dormant, which could have a cascade effect on the water and send it into a mini-cycle. Depending upon the water volume of the sump and the ambient room temperature, it's unlikely that overnight could cause this to any significant degree.
Chaeto (or any macro algae) would be the most susceptible to challenges. Mostly the problem would be lighting. Assuming that you've not lost the light source, then it'll depend upon how much your chaeto normally churns. If not at all, you likely won't be impacted much. If your chaeto normally spins from your flow, then it may get some die-off in the lower sections, but likely not much, I'd not think. Not overnight, at least. Adding a temporary powerhead into the fuge could solve that problem.
