Effectivement, un défi sans participants prend fin immédiatement sans résolution.
FAQ (3.33) No Characters in a Challenge
If, at any time after a challenge begins and attackers are declared, there are no participating characters in that challenge, the challenge ends immediately without resolution.
Par contre, on ne peut pas gagner un défi avec 0 de force (à 0 contre 0, personne ne gagne).
Coreset
A challenge may not be won by an attacker or defender whose total STR is less than 1, or who has no participating characters when the challenge is resolved.