I don't know what it is about black walnut, but they do seem more prone to lightning strikes than any other tree. I'm guessing it has something to do with their bark when it's wet making a solid nice circuit of water down to the ground. Anyway, I can think of about a dozen lightning damaged trees in my area and almost all are walnuts. It's still pretty darn unlikely that any one individual tree is going to be hit though, and if it is hit, I'm not sure it would ruin the log since the damage is all on the bark. You would just have to cut the tree down soon after it was hit because it won't heal well (or at all).