UAlsAnimationInstance::RefreshVelocityBlend may produce NaN when velocity is zero?

[watsonsong]

When the character is not moving, the RelativeVelocityDirection may get a zero value. Which lead to the TargetVelocityBlend contains NaN. It could be better check this. The NaN may pass to ForwardAmount on some platform. It may lead the animation has some error.