The entire reason that there were no people there was because Azula had banished tme in a fit of psychotic paranoia. If therehad been people there, then she would presumably have been sane, and on much more equal footing with Zuko. So either way.
As far as the rules go, I view it as being similar to a game of tennis. If one player hits the ball out, but before it can go out, the other player accidently knocks it back in, the ball stays in play. The fact that it was going out is irrelevant - the other player chose to hit it back in, the game goes on. Likewise, Zuko chose to intercept the lightning bolt, and keep the "game" going. To his detriment.