Fix last seen not set error on admin user page

app/templates/admin/user.html.j2

@@ -41,7 +41,7 @@
             <li>Id: {{ user.id }}</li>
             <li>Hashid: {{ user.hashid }}</li>
             <li>Member since: {{ user.member_since.strftime('%Y-%m-%d') }}</li>
-            <li>Last seen: {{ user.last_seen.strftime('%Y-%m-%d') }}</li>
+            <li>Last seen: {% if user.last_seen %}{{ user.last_seen.strftime('%Y-%m-%d') }}</li>{% endif %}
           </ul>
         </div>
         <div class="card-action right-align">