[fix] search: crash when no engines are used to search

searx/templates/simple/elements/engines_msg.html

@@ -1,5 +1,5 @@
 <div id="engines_msg">
-  {% if not results and not answers %}
+  {% if (not results and not answers) or not max_response_time %}
   <details class="sidebar-collapsable" open>
     <summary class="title" id="engines_msg-title">{{ _('Messages from the search engines') }}</summary>
   {% else %}