Always render Null Objects with 0% Transparency
I often use Null Objects within my compositions- and its very easy to forget to switch them off before you render- leaving white squares where they have been used in you render.
So why not as an overall rule just always render them with 0% transparency? There doesnt seem to be any reason for Null objects to be rendered so it makes sense.
(You can obviously set them to 0% in the composition- but it often actually helps for them to be visible whilst constructing your comps)
the dudes commented
Turning them into guide layers would just do this. I do second your idea though. Null objects could stay null objects, because now, they're technically just solids.