O_O
I've never seen a rare mob not drop an item. Outside of it being replaced with a gem of some sort, my memory is that they always drop at least 1 item.
My bad, i was looking for "rare" in the Maxroll page, since you used that term.
Only to realise that... there are no such thing as "rare" mobs XD
Regarding champions and uniques, they do have guarenteed drops.
Champions can drop gold instead of a non-potion item though.