When you display a sidebar it's great to check if that sidebar actually has widgets inside. You may end up with an empty rectangle if you did not add any widgets to a sidebar. There are two methods to actually do this.

You can output buffer the sidebar and see if it has any text inside. If length is zero don't display it. Or ... you can use the missing is_sidebar_active(). It's actually missing but here is how it should look like. This function will check if a sidebard has widgets inside and, if it's empty, you can skip it.

Wordpress Widgets API is_sidebar_active() Helper

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The function takes as parameter a number or a literal sidebar ID (which I prefer) and tells you if it has widgets or is empty and ... if it's empty ... you should not dynamic_sidebar it. So use literal sidebar IDs make sure you use an ID when you call register_sidebar.

Use it wisely ... I know I do :)