I think a major error in the rules has been discovered over at BGG. The rules, as written would allow an attacker, who through effects has a zero strength rating, to still initate an attack. If the defender also played a reduced to zero character the attacker would win, but would also get a power bonus for a defender defending with a strength less than 1. This is clearly addressed in AGoT on page 14, but has not been carried over to GoT.
A challenge may not be won by an attacker or a major defender whose total STR is less than1, or who has no participating characters when the challenge is resolved.