Fixed wrong sum calculation if no feedback is available

sumScore may be undefined if there are submissions but with no feedback. Therefore sumScore will be displayed nothing instead of 0.

frontend/src/components/student/SubmissionList.vue

@@ -31,7 +31,7 @@
       color="info"
       value="true"
     >
-      You reached <b>{{ sumScore }}</b> of <b>{{ sumFullScore }}</b> possible points ( {{ pointRatio }}% ).
+      You reached <b>{{ sumScore }}</b> of <b>{{ sumFullScore }}</b> possible points ({{ pointRatio }}%).
     </v-alert>
   </div>
 </template>
@@ -74,7 +74,7 @@ export default {
   },
   computed: {
     sumScore () {
-      return this.submissions.map(a => a.feedback && a.feedback.score).reduce((a, b) => a + b)
+      return this.submissions.map(a => a.feedback && a.feedback.score).reduce((a, b) => a + b) || 0
     },
     sumFullScore () {
       return this.submissions.map(a => a.type.fullScore).reduce((a, b) => a + b)