[ISSUE #2835] Fix an uncaught exception when accessing restricted namespace in authentication mode.

[ljhrot]

#2835

What is the purpose of the change

Fix an uncaught exception when accessing restricted namespace in authentication mode.

Brief changelog

Fix an uncaught exception when accessing restricted namespace in authentication mode.

Verifying this change

1. 修改配置 nacos.core.auth.enabled=true 开启鉴权。
2. 使用 nacos/nacos 登陆，创建新的 namespace。
3. 创建新的账号，只赋予上面创建的 namespace 管理权限，这个时候新的账号是没有 public 和其他 namespace 的管理权限的。
4. 使用上面创建的账号登陆，访问没有权限的 namespace，提示 没有权限！ 即为正常。

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