My group (and I agree) ruled that if you're using the Dervish Dance feat, you're actively using that arm (empty hand) in order to make your dance. By extension, it means you're using that arm in order to gain its benefit, so you're not effectively using the arm to defend yourself with the buckler strapped onto it.
You could houserule it's okay to carry a light object in the empty hand as long as it doesn't get used to do anything while making Dervish Dance attacks (e.g. it will bestow no benefit whatsoever until your next turn).