fix: handle null referrer in pixel tracking route

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

src/app/(collect)/p/[slug]/route.ts

@@ -47,7 +47,7 @@ export async function GET(request: Request, { params }: { params: Promise<{ slug
     payload: {
       pixel: pixel.id,
       url: request.url,
-      referrer: request.headers.get('referer'),
+      referrer: request.headers.get("referer") || undefined,
     },
   };