Does Blue Teemo trigger set off even if the controlling player can't target Ruin Runner?

Here's the ruling.

No — the trigger itself goes on the chain, but if Ruin Runner is the only enemy unit there, the ability is removed during finalization and does nothing.

Here's how it works:

1. Teemo defends → the trigger condition is met and his ability enters the chain as a Pending Item.
2. During finalization (Rule 402), the controller must choose an enemy unit here as a target. Per Rule 355.10, "choose an enemy unit" means targeting that unit.
3. Ruin Runner can't be chosen by enemy spells and abilities, so it's not a legal target. If Ruin Runner is the only enemy unit at that battlefield, there are no legal options.
4. Rule 402.3 says: "If there are not enough options to make legal choices for a Triggered Ability that has been put on the chain, remove it from the Chain now. It ceases to be a Pending Item but never becomes a Chain Item."

So the trigger "fires" (enters the chain as pending), but it gets removed without resolving. No cards are revealed, no damage is dealt, and this isn't considered the ability being countered — it simply can't proceed.

If there are other enemy units at the battlefield besides Ruin Runner, the controller must choose one of those legal targets instead (Rule 402.3.b), and the ability resolves normally against that unit.