Make the initially visible block in the expander optional

If there is no "general" block defined in the expander, it just won't show.

templates/includes/expander.twig

         <div class="expandable{% if expanded %} expanded{% endif %}">
+          {% if block('general') is defined %}
           <div class="general">
-            {%- block general%}{% endblock %}
+            {{- block("general") }}
 
           </div>
+          {% endif %}
           <a tabindex="0" class="expander"></a>
           <div class="content">
             {%- block content %}{% endblock %}
 
           </div>
-        </div>
+        </div>
\ No newline at end of file