fix(router/atc): reset rebuilding flag if route has no id

[chronolaw]

Summary

The variable rebuilding is a flag to prevent duplicate router building action,
if any error occurs it should be set to false,
or else the router will never be rebuilt again.

KAG-3857

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 #[issue number]

[team-gateway-bot]

Successfully created cherry-pick PR for master:

• https://github.com/kong/kong-ee/pull/8495