Show WordPress widget area only if it exists

If there is no widgets in your widget area you don’t want to display a sidebar container element, like UL tag in the example.
You can use is_active_sidebar('sidebar-name-or-id') function to check, whether the sidebar contains any widgets or not.

Instructions
Wrap this code around your widget container

Related links:
http://codex.wordpress.org/Function_Reference/is_active_sidebar