refactor zone retrieval logic to unhandle unknown zones

[krishnamuppaneni]

This PR refactors the Cloudflare Worker's zone handling logic in worker.js to prevent exceptions when requests for unknown zones are processed.

Added explicit handling for cases where no zone is found for a request:

• If getZoneFromReqURL() returns null/undefined, the worker now logs "No zone found for this request" and early exits
• This prevents further processing for domains that aren't configured
• This also prevents unnecessary KV reads for unknown zones and will early exit

[rr404]

Looks ok @blotus what do you think ?