Automatic item modification by age isn't able to match on a null value, so we can't set a rule for something like "all items more than 60 days old that don't have any collection code." That'd be handy. Looks like bug 18752 made it so we could set a blank as our substitution value, but doesn't help on the match value. The issue may be that Koha ends up looking for an empty string rather than for a null.
Being able to set a condition to match on a null value would be a very helpful enhancement to this tool.
I think this sounds like a useful enhancement. I would also like to see more options like "!=" or "IS NOT NULL" Like in this mockup -> https://pasteboard.co/JIoatdC.gif