Do not mutate topic if unchanged

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

@@ -63,10 +63,13 @@ export default {
   },
   methods: {
     saveTopic() {
-      this.createOrPatch([{
-        id: this.documentation.id,
-        topic: this.topic,
-      }]);
+      if (this.topic) {
+        this.createOrPatch([{
+          id: this.documentation.id,
+          topic: this.topic,
+        }]);
+        this.topic = "";
+      }
     },
     truncate(str) {
       return str ?