title: Nuxt-Router: Slug in URL should not be optional
body: <p>My directory structure looks like that
  /pages/request/guest/_slug/index.vue</p>
  
  <p>If I call /request/guest/test-slug/ then Nuxt opens the index.vue file as expected.</p>
  
  <p>But /request/guest/ without a slug also opens the same index.vue file. </p>
  
  <p>Why is that? It should return an 404. </p>