How does the game handle empty TCs?

[Kurrelgyre]

I am working on updating my Excel item drop calculator for 1.10. Many of the 'system-generated' treasure classes such as the bow and mele classes are empty since there are no items at those levels. Does the game never spawn the empty classes in the first place, does it do a nodrop if it picks an empty class, or does it fall back to the first TC below that contains items?

"Don't sweat the petty things, it's more fun to pet the sweaty things."