The only game that matters
You must have a total strength greater than 0 to win a challenge. In your example, no one would win the challenge, and no one would lose.
But because there is still a character participating, the challenge would go to resolution (if Robb was removed from the challenge entirely, the challenge would end immediately).
Khudzlin said:
But because there is still a character participating, the challenge would go to resolution (if Robb was removed from the challenge entirely, the challenge would end immediately).
Right. With the resolution being nullified because no one had a STR 1 or higher character involved in the challenge.
Careful with the "resolution being nullified" language. The resolution still HAPPENS, it just resolves with no winner and no loser. So anything that requires a winner or a loser (like claim, unopposed, and renown) don't resolve successfully. However, the challenge resolved successfully, so any "after a challenge resolves"-type stuff would still happen. (People might think otherwise if resolution were considered "nullified.")