docs: fix session.user is possibly undefined. (#7058)

fixed the code snippet for the example to consume session via hooks. Threw an error earlier stating 'session.user' is possibly 'undefined'.

docs/docs/getting-started/oauth-tutorial.mdx

@@ -128,7 +128,7 @@ import { useSession, signIn, signOut } from "next-auth/react"
 
 export default function CamperVanPage() {
   const { data: session, status } = useSession()
-  const userEmail = session?.user.email
+  const userEmail = session?.user?.email
 
   if (status === "loading") {
     return <p>Hang on there...</p>