Add default text for empty submission list

client/src/components/AssignmentWithSubmissions.vue

@@ -1,6 +1,7 @@
 <template>
   <div class="assignment-with-submissions">
     <h1 class="assignment-with-submissions__title">{{assignment.title}}</h1>
+    <p v-if="!assignment.submissions.length">Zu diesem Auftrag sind noch keine Ergebnisse vorhanden</p>
      <router-link
       :to="submissionLink(submission)"
       v-for="(submission, index) in assignment.submissions"