Mathematically speaking, the zero vector cannot be normalized. Its length will always remain 0.
For given vector v = (v1, v2, ..., vn) we have: ||v|| = sqrt(v1^2 + v2^2 + ... + vn^2). Let us remember that a normalized vector is one that has ||v||=1.
So for v = 0 we have: ||0|| = sqrt(0^2 + 0^2 + ... + 0^2) = 0. You can never normalize that.
Also important to note that to ensure consistency, you should not return NaN or any other null value. The normalized form of v=0 is indeed v=0.