fix(cors): treat empty Origin header same as missing header

[dingjiayi]

Summary

This change ensures that an empty Origin header (Origin: "") is treated
the same way as a missing Origin header, fixing inconsistent behavior
between these two cases.

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 #14352

[StarlightIbuki]

Overall LGTM. Not a blocker but please take a look

[StarlightIbuki]

Perhaps this way it's simpler?

Suggested change

	if req_origin and req_origin ~= "" then
	if req_origin == "" then
	req_origin = nil
	end
	if req_origin then

[dingjiayi]

Hi @StarlightIbuki ，

Thanks a lot for reviewing this PR and for your suggestion!

I agree, your proposed change does make the conditional logic simpler and the code cleaner. That's a good direction.

One minor point I noticed is that this approach would convert an empty string req_origin ("") into nil. While this doesn't have a functional impact on the current plugin's behavior, it does lead to a subtle loss of information by removing the distinction between an explicitly empty origin and a non-existent one (nil).

Inspired by your comment aiming for simplification, what do you think about encapsulating this check into a small helper function instead? This way, we can keep the main logic clean while preserving the original values.

Something like this:

local function is_valid_origin(origin)
  return origin and origin ~= ""
end

We could then use if is_valid_origin(req_origin) then ... in the configure_origin function.

Let me know your thoughts! Thanks again for the feedback.