bug: bind the route to the public-api plugin, upstream is a required option in the dashboard, but it is not actually needed

### Current Behavior

通过dashboard使用public-api暴露接口时，只需要指定uri即可，但是通过dashboard的route里添加是upstream为必选项，与实际接口要求不匹配，希望可以优化dashboard前端。

### Expected Behavior

使用dashboard，在route里绑定public-api插件时，route的upstream为非必选项。

### Error Logs

页面报错，没有日志。

### Steps to Reproduce

1.登陆dashboard
2.添加route，绑定public-apid对metrics接口进行暴露
3.route不填写upstream信息
4.提交保存
5.页面提示upstream为必选项，导致保存失败

### Environment

- APISIX version (run `apisix version`):3.16
- Operating system (run `uname -a`):redhat 9
- OpenResty / Nginx version (run `openresty -V` or `nginx -V`):1.27.1.2
- etcd version, if relevant (run `curl http://127.0.0.1:9090/v1/server_info`):3.x
- APISIX Dashboard version, if relevant:3.16
- Plugin runner version, for issues related to plugin runners:na
- LuaRocks version, for installation issues (run `luarocks --version`):na

Start in the dashboard route creation or editing form and trace the validation applied when the public-api plugin is bound. Reproduce the reported metrics route without an upstream, then verify that saving succeeds while routes that need an upstream still validate correctly.

Written by the indexing model from the issue text.