RDF template returns server error if package has no extras

In very special situations, pkg_dict hasn't such key as 'extras'.
it results in server error, because rdf template uses `None` as
fallback value.

Replaced it with empty list - it's more logically

ckan/templates/package/read.rdf

@@ -59,7 +59,7 @@
     <dct:rights rdf:resource="{{c.pkg_dict['license_url']}}"/>
     {% endif %}
 
-    {% for extra_dict in c.pkg_dict.get('extras',None) %}
+    {% for extra_dict in c.pkg_dict.get('extras',[]) %}
         <dct:relation>
           <rdf:Description>
             <rdfs:label>{{extra_dict.get('key','')}}</rdfs:label>