修复顶级菜单查询不到的bug

src/main/java/cn/stylefeng/guns/modular/system/service/MenuService.java

@@ -114,7 +114,7 @@ public class MenuService extends ServiceImpl<MenuMapper, Menu> {
 
         //获取menuId的code
         String code = "";
-        if (menuId != null) {
+        if (menuId != null && menuId != 0L) {
             Menu menu = this.getById(menuId);
             code = menu.getCode();
         }