修复当接口返回的品类数据为空时，搜索列表会报错的bug

Code Review By Rock Zhang

@@ -113,6 +113,11 @@ class SearchController extends AbstractAction
             $classNames = Category\ClassModel::getClassNames();
 
             do {
+                // 品类名称为空时跳出
+                if (empty($classNames)) {
+                    break;
+                }
+                
                 /* 精确查一级品类 */
                 $sorts = array_keys($classNames['first'], $query, true);
                 if (isset($sorts[0])) {