Guard useSwr() to avoid sending invalid request (#20671)

examples/api-routes-rest/pages/user/[id].js

@@ -5,7 +5,10 @@ const fetcher = (url) => fetch(url).then((res) => res.json())
 
 export default function User() {
   const router = useRouter()
-  const { data, error } = useSwr(`/api/user/${router.query.id}`, fetcher)
+  const { data, error } = useSwr(
+    router.query.id ? `/api/user/${router.query.id}` : null,
+    fetcher
+  )
 
   if (error) return <div>Failed to load user</div>
   if (!data) return <div>Loading...</div>