The same bug exists in the link route (q/[slug]) where
request.headers.get("referer") returns null for requests without
a Referer header, causing silent Zod validation failure.
request.headers.get("referer") returns null when the header is absent
(e.g. email clients). The Zod schema uses .optional() which accepts
undefined but rejects null, causing silent validation failure and
pixel events not being recorded.
Fixes#4028
juanisidoro