对问题的描述非常简单，但显而易见的解决方案是仅在查询不为空时输出小部件数据：

class cond_widget extends WP_Widget {

  public function __construct() {
    // widget actual processes  
    parent::__construct(
        \'test\', // Base ID
        \'test\', // Name
        array( \'description\' => __( \'test\', \'foo\'))
    );
  }

  public function widget( $args, $instance ) {

    $qry = $wpdb->get_var(\'whatever\');
    if (!empty($qry)) {
      echo $args[\'before_widget\'];

      if (!empty($instance[\'title\'])) {
        $title = $instance[\'title\'];
        echo $args[\'before_title\'].$title.$args[\'after_title\'];
      }

      echo \'<ul class="recent_pages">\'; 

      echo \'</ul>\';
      echo $args[\'after_widget\'];
    }
  }

}
add_action( 
  \'widgets_init\', 
  function(){
     register_widget( \'cond_widget\' );
  }
);