bug: APISIX rewrites yaml configuration file to cause restarting errors

### Current Behavior

APISIX generates and writes a configuration file when the Admin API key is empty, which causes the configuration file to be exported from memory and encoded in YAML and then written.
https://github.com/apache/apisix/blob/master/apisix/core/id.lua#L124-L133
During this process, comments in the configuration file will be lost, and some will be generated containing `...` ending, which will result in an error on the next startup.

### Expected Behavior

Configuration files should not be rewritten in any way. If the Admin API key is empty, we need to write it to the logs and also warn the user there that you need to configure your own key, otherwise every reboot will cause the key to change.

### Error Logs

No need

### Steps to Reproduce

1. Set `deployment.admin.admin_key` in the configuration file to the empty string
2. Start APISIX
3. Check config.yaml

### Environment

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