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.

I got this response from FFG:

"Thanks for bringing this to our attention.

I would go with answer 3 since there were NO defenders. An undefended attack is an undefended attack regardless. However, had there BEEN a defender with a strength of 0, I would consider that having successfully defended from the -1 attack.

At least, that's how I interpret the rule…

I'm afraid, regardless of the response I got from FFG, this shows that they will have to address this in a FAQ.  The answer would be "3", that even a negative strength attacker can initate a challenge.  And not only would he win, but he would get the undefended power bonus.

Strength cannot be less than 0.  Straight from the FAQ

http://fantasyflightgames.com/ffg_content/agotlcg/support/10-11/AGoT-LCG-FAQ_3.4_Low_res.pdf