Properly call `super()` in overridden blocks in builtin templates,

even if the blocks are empty in the builtin layout.html they may contain
something in an overridden one.

sphinx/templates/genindex-single.html

@@ -42,4 +42,5 @@
    {%- endfor %}</p>
 
    <p><a href="{{ pathto('genindex-all') }}"><strong>{{ _('Full index on one page') }}</strong></a></p>
+   {{ super() }}
 {% endblock %}

sphinx/templates/genindex-split.html

@@ -26,4 +26,5 @@
 
    <p><a href="{{ pathto('genindex-all') }}"><strong>{{ _('Full index on one page') }}</strong></a></p>
 {% endif %}
+   {{ super() }}
 {% endblock %}

sphinx/templates/genindex.html

@@ -53,4 +53,5 @@
 
    <p><a href="{{ pathto('genindex-all') }}"><strong>{{ _('Full index on one page') }}</strong></a></p>
 {% endif %}
+   {{ super() }}
 {% endblock %}

sphinx/templates/modindex.html

@@ -1,6 +1,7 @@
 {% extends "layout.html" %}
 {% set title = _('Global Module Index') %}
 {% block extrahead %}
+{{ super() }}
 {% if builder != 'htmlhelp' and collapse_modindex %}
     <script type="text/javascript">
       DOCUMENTATION_OPTIONS.COLLAPSE_MODINDEX = true;