about page: dont create an empty p if no message
<div id="explanation">
<p>This website is powered by <a href="https://github.com/jupyterhub/binderhub">BinderHub</a> v{{ binder_version }}.
</p>
+ {% if message %}
<p>
{{ message | safe }}
</p>
+ {% endif %}
</div>
</div>
{% endblock header %}
