chore(docs): turn on strictNullChecks (#36180)

strictNullChecks was off for docs, which lets errors slip through and
leads to incorrect required/optional typing on Zod-inferred types. This
PR enables strictNullChecks and fixes all the existing violations.

apps/docs/layouts/ref/RefSubLayout.tsx

@@ -89,7 +89,7 @@ const StickyHeader: FC<StickyHeader> = ({ icon, ...props }) => {
 
   // we're serving search bots a different file (/crawlers/[...slug])
   // and need to modify content to suit that
-  const isCrawlerPage = router.route.includes('/crawlers/[...slug]')
+  const isCrawlerPage = router?.route.includes('/crawlers/[...slug]') || false
 
   const { ref } = useInView({
     threshold: 1,