Fix admindocs on Python 3, where None cannot be sorted with strings.

This fixes two tests in admin_views which were failing on Python 3, but only if
the tests were run with docutils installed.

django/contrib/admindocs/templates/admin_doc/template_filter_index.html

@@ -22,7 +22,7 @@
     <h2>{% firstof library.grouper "Built-in filters" %}</h2>
     {% if library.grouper %}<p class="small quiet">To use these filters, put <code>{% templatetag openblock %} load {{ library.grouper }} {% templatetag closeblock %}</code> in your template before using the filter.</p><hr />{% endif %}
     {% for filter in library.list|dictsort:"name" %}
-    <h3 id="{{ library.grouper|default_if_none:"built_in" }}-{{ filter.name }}">{{ filter.name }}</h3>
+    <h3 id="{{ library.grouper|default:"built_in" }}-{{ filter.name }}">{{ filter.name }}</h3>
     {{ filter.title }}
     {{ filter.body }}
     {% if not forloop.last %}<hr />{% endif %}
@@ -43,7 +43,7 @@
     <h2>{% firstof library.grouper "Built-in filters" %}</h2>
     <ul>
     {% for filter in library.list|dictsort:"name" %}
-        <li><a href="#{{ library.grouper|default_if_none:"built_in" }}-{{ filter.name }}">{{ filter.name }}</a></li>
+        <li><a href="#{{ library.grouper|default:"built_in" }}-{{ filter.name }}">{{ filter.name }}</a></li>
     {% endfor %}
     </ul>
 </div>

django/contrib/admindocs/templates/admin_doc/template_tag_index.html

@@ -22,7 +22,7 @@
     <h2>{% firstof library.grouper "Built-in tags" %}</h2>
     {% if library.grouper %}<p class="small quiet">To use these tags, put <code>{% templatetag openblock %} load {{ library.grouper }} {% templatetag closeblock %}</code> in your template before using the tag.</p><hr />{% endif %}
     {% for tag in library.list|dictsort:"name" %}
-    <h3 id="{{ library.grouper|default_if_none:"built_in" }}-{{ tag.name }}">{{ tag.name }}</h3>
+    <h3 id="{{ library.grouper|default:"built_in" }}-{{ tag.name }}">{{ tag.name }}</h3>
     <h4>{{ tag.title|striptags }}</h4>
     {{ tag.body }}
     {% if not forloop.last %}<hr />{% endif %}
@@ -43,7 +43,7 @@
     <h2>{% firstof library.grouper "Built-in tags" %}</h2>
     <ul>
     {% for tag in library.list|dictsort:"name" %}
-        <li><a href="#{{ library.grouper|default_if_none:"built_in" }}-{{ tag.name }}">{{ tag.name }}</a></li>
+        <li><a href="#{{ library.grouper|default:"built_in" }}-{{ tag.name }}">{{ tag.name }}</a></li>
     {% endfor %}
     </ul>
 </div>

django/contrib/admindocs/views.py

@@ -62,7 +62,7 @@ def template_tag_index(request):
             for key in metadata:
                 metadata[key] = utils.parse_rst(metadata[key], 'tag', _('tag:') + tag_name)
             if library in template.builtins:
-                tag_library = None
+                tag_library = ''
             else:
                 tag_library = module_name.split('.')[-1]
             tags.append({
@@ -97,7 +97,7 @@ def template_filter_index(request):
             for key in metadata:
                 metadata[key] = utils.parse_rst(metadata[key], 'filter', _('filter:') + filter_name)
             if library in template.builtins:
-                tag_library = None
+                tag_library = ''
             else:
                 tag_library = module_name.split('.')[-1]
             filters.append({