fix(core): should_warn checks if kong global has been defined

[nowNick]

Summary

After merging: https://github.com/Kong/kong/pull/13069 it was revealed in some tests that kong global might not have been initialized yet. This PR checks first if kong global is defined as it's done in a few other places in this module. It's just to make sure unit test work.

Checklist

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

Issue reference

KAG-4515

[team-gateway-bot]

Successfully created cherry-pick PR for master:

• https://github.com/kong/kong-ee/pull/9601