fix in stats comp when mean was null

frontend/src/components/CorrectionStatistics.vue

@@ -7,7 +7,11 @@
         <ul class="inline-list mx-3">
           <li>Submissions per student: <span>{{statistics.submissions_per_student}}</span></li>
           <li>Submissions per type: <span>{{statistics.submissions_per_type}}</span></li>
-          <li>Curr. mean score: <span>{{statistics.current_mean_score.toFixed(2)}}</span></li>
+          <li>Curr. mean score:
+            <span>
+              {{statistics.current_mean_score === null ? 'N.A.' : statistics.current_mean_score.toFixed(2)}}
+            </span>
+          </li>
         </ul>
         <v-divider class="mx-2 my-2"></v-divider>
         <div v-for="(progress, index) in statistics.submission_type_progress" :key="index">