I can't seem to find an explicit statement either in game, or in the forums from the devs that participation is all that is required, outside of the in game message that says (paraphrasing) "you didn't participate so you get nada". I was always under the impression that banners had to be earned,
But in the situation where you don't set defenses, and you only lose on offense, you would not gain banners. However, you have participated. Do you get rewards in this situation?