If the opponent only has the Active Pokémon left and no benched one, can the Tri Hazard GX attack still be used even if there is nobody on the bench?
There's an official ruling for this attack in The Rulings Compendium Black and White (the official ruling source):
Tri Hazard GX (Alolan Muk-GX - Burning Shadows)
Q. If my opponent has no Benched Pokemon, can I still use Alolan Muk-GX's "Tri Hazard-GX" attack? A. You can use it, but it won't have any effect, and it will count as having used your GX attack for the game. (Burning Shadows FAQ; Aug 3, 2017 TPCi Rules Team)
You can use the attack, but it won't have any effect except it counts as you've used your GX attack for that game. Switching 1 of your opponent's Benched Pokémon with their Active one is a required step to do the effect.