how to hide when ads in blank

[may820806]

I have some ad-unit
but it's not always pushing ads
sometimes it will be blank, and it's okay,
the question is, can I hide the component when there'is no ad?
'cause now it's a white block there
and i don't want to show when it is empty
you know, i want the content down below go up.
is there a config can do this?
(I tired collapseEmptyDivs: true, but it's not working?)
thanks.