There are several items with no bonuses (those are MI and bugged), but this is a different matter.
Also unique jewelry and artifacts are empty and don't drop, at least the vast majority.
For your example, it's not a bug, it's just the "of the Tinkerer" suffix that allows to socket two relics instead of one.
Everyone asks the same question everywhere, so the devs should (or are planning to ?) add a description under the name of the item to state that.
Maybe there's a text once you have socketed one relic, but they should add one when there's nothing socketed too.
I think that you can't socket twice the same relic in the same item.