Other than common sense? Once you actually reach the 'Declare Defenders' step, there isn't any participating unit to declare a defense against. So who or what are you defending? My vote is there is no defense to declare, so you won't have any participating units, and with that, there is no edge allowed.
Beyond that, if defenders were permitted (ie the strike doesnt just fizzle) but none were declared, the challenge would be unopposed and the LS would do 1 damage to the engaged objective…even though no one attacked?
Then this one lacks comon sense, nor is there any "intent" to stop the battle either. Call it poor planning on the attackers side. There's only two reason to even declare defenders at that point, tactics or targeted strike so it would be usefull to not have to worry about the one attacker to get these effects through.
It would not grant unopposed bonus, as the beginning of the second paragraph reads "If at least one attacking unit has survived…"
I'm sure there will be an official ruling on this at some point, but based on past precedents in other LCG's I'm fairly confident I know the answer. From the A Game of Thrones 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.
I assume that Star Wars will follow the same model. If, prior to declaring defenders, all attacking characters are rendered non-participants -- they're killed, returned to hand, or removed from the challenge by other effects -- then the conflict immediately ends without resolution, and the attacker has used their opportunity to engage the declared objective.