[debugtoolbar] Do not render CW controller panel when no controller got collected

This can happen on pure pyramid applications for instance.

cubicweb/pyramid/debug_toolbar_templates/cw.dbtmako

+% if controller:
+
 <h3>Controller</h3>
 
 <table class="table table-striped table-condensed">
@@ -61,3 +63,6 @@
     <li><a href="/cwsource">data sources</a></li>
     <li><a href="/siteinfo">Site information</a></li>
 </ul>
+% else:
+<p>No controller store got collected.</p>
+% endif