Hiding A Div If A Variable Is Empty ?

I'm using a Javascript gallery (runs on Prototype library) that populates a caption field if the user has entered a caption for an image.

I need a little bit of code to hide the caption's div entirely if the user hasn't entered a caption for a particular image (var 'caption' is empty).

It's the bottom-right white box below the image, you'll have to click over to the second page of the gallery (NEXT) to see an example of an empty one that I want to hide:

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Here's the CSS:

Here's the HTML:

I'm sure this isn't that hard, but I've spent more time trying to get that caption box to hide than setting up the gallery.