Don't show None to the user if no topic is set

core/templates/partials/rule-list.html

@@ -23,7 +23,7 @@
       <td>{{ item.user }}</td>
       <td>{{ item.name }}</td>
       <td>{{ item.priority }}</td>
-      <td>{{ item.topic }}</td>
+      <td>{{ item.topic|default_if_none:"—" }}</td>
       <td>
         {% if item.enabled %}
           <span class="icon">