I don't think that Asatru has any specific opinions on fapping, per se. Some people will point to the cultural practice of abstinence before adulthood of Scandinavian societies; whether this is religiously-based or cultural is up for debate.
The best thing to do would be look at various arguments and statistics and make a decision from there. Some people say that fapping lowers testosterone, some say it doesn't. Some say that porn can lead to addiction or other problems, some say it doesn't. Look at both sides, try both sides, make a decision.
Personal opinion, fapping is fine, but excess fapping or porn is degenerate. I see nothing wrong with "using your imagination".