update ZR.Service/System/SysMenuService.cs.

菜单管理查询为空时报错，增加了查询为空时返回一个默认值；

Signed-off-by: xbzhu <5430389+xbzhu@user.noreply.gitee.com>

ZR.Service/System/SysMenuService.cs

@@ -1,4 +1,4 @@
-using Infrastructure.Attribute;
+using Infrastructure.Attribute;
 using Infrastructure.Extensions;
 using SqlSugar;
 using System;
@@ -41,7 +41,7 @@ namespace ZR.Service
                 var userRoles = SysRoleService.SelectUserRoles(userId);
                 menuList = SelectTreeMenuListByRoles(menu, userRoles);
             }
-            return menuList;
+            return menuList ?? new List<SysMenu>();
         }
 
         /// <summary>