"At the end of the environment turn, this card deals each target 2 fire damage. Each hero must either destroy 1 of their ongoing cards or take 2 more fire damage."
If I control no ongoing cards, can I still choose to destroy 1? I assume not, but I figure, it is still a choice...
No, you actuallly need to destroy one to avoid the damage
The only time you can lose "nothing" of something to avoid/trigger an effect is discarding your hand when you have zero cards, because a hand of zero cards is still a hand (Toxic Seaweed in Atlantis is an example of this). Otherwise, you must always discard/destroy the specified number of cards in order to achieve whatever it is.
Another Reality's Debt is a similar example - you may destroy all your cards in play if you have none, or discard an empty hand.