Fix Tree.resolve_polytomies() to not fail when a polytomy occurs a the root

I believe this is will preserve the past randomness while allowing polytomies to be resolved when they are at the root (e.g. star tree). I was trying to randomly perturb some trees using this method and it would fail when the root was chosen as the 'next_sib'. I could be wrong, but I believe shuffling the children as I did on line 1340 removes the need to include 'node' in attachment points.