There are no bucklers and there is no purpose to single wielding outside of a single Legendary Rapier you get in Act 3.

Beyond that, there's no purpose for having an empty hand. Spells don't require somatic components so you can have your hands full no problem. Duel wielding doesn't penalize your attack. Duelling fighting style still works even when using a Shield.

The only time an "Empty" hand makes any sense is when using a Versatile weapon to make it 2 handed or when using the aforementioned Legendary Rapier. Otherwise you're just missing out on free AC/stats.

Now... How impactful is it to not utilize your off-hand? Not very. The game isn't difficult enough to have a character being slightly suboptimal have a major impact, even more so if you're not playing on Honour Mode or Tactician difficulties.