For a non-empty finite complex number set $A$, define the "Tao root" of $A$ as $\left|\sum_{z\in A} z \right|$. Given the integer $n\ge 3$, let the set $$U_n = \{\cos\frac{2k \pi}{n}+ i\sin\frac{2k \pi}{n}|k=0,1,...,n-1\}.$$Let $a_n$ be the number of non-empty subsets in which the Tao root of $U_n$ is $0$ , $b_n$ is the number of non-empty subsets of $U_n$ whose Tao root is $1$. Compare the sizes of $na_n$ and $2b_n$.