Make subject chip work with missing subject

aleksis/apps/alsijil/frontend/components/documentation/LessonInformation.vue

@@ -3,7 +3,10 @@
     {{ $d(toDateTime(documentation.datetimeStart), "shortTime") }} –
     {{ $d(toDateTime(documentation.datetimeEnd), "shortTime") }}
     {{ documentation.course?.name }}
-    <subject-chip :subject="documentation.subject" />
+    <subject-chip
+      v-if="documentation.subject"
+      :subject="documentation.subject"
+    />
   </v-card-text>
 </template>