fix(admin): reject AdminAPI call with empty tags

[nowNick]

Summary

When querying entities that have tags assigned a user can send a request with tags parameter. Previously sending tags='' (empty string or explicit nil) resulted in 500 error. This commit changes that so that it returns 400 error as empty explicit tags are not allowed

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-5496 Fix #13591