Prints an empty page.
This function is used to print out a bare minimum empty page which still has the scripts and styles necessary in order to trigger the overlay to close.
function overlay_deliver_empty_page() {
$empty_page = '<html><head><title></title>' . drupal_get_css() . drupal_get_js() . '</head><body class="overlay"></body></html>';
print $empty_page;
drupal_exit();
}