fix(core): set worker ID of privileged agent to -1

[outsinre]

Summary

By default, ngx.worker.id() returns nil for the privileged agent. Now Fall back to -1 as the worker ID of privileged agent worker to avoid error. Keep consistent with code like

kong/kong/globalpatches.lua

Line 427 in 0ff50d5

id = ngx.worker.id() or -1

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #FTI-5707

[ms2008]

There are still several things that haven't been changed, like here

kong/kong/runloop/handler.lua

Line 638 in 0ff50d5

local worker_id = ngx_worker_id()

Please find out where all the calls are made with the keyword worker_id.

[outsinre]

ngx_worker_id

Done.