Handle products without a category in GA subission

Without this change, the category is "None"

oscar/templates/oscar/partials/google_analytics_transaction.html

@@ -15,7 +15,7 @@ _gaq.push(['_addTrans',
         '{{ order.number }}',
         '{{ line.partner_sku|escapejs }}',
         '{{ line.title|escapejs }}',
-        '{{ line.category|escapejs }}',
+        '{{ line.category|default:"Uncategorised"|escapejs }}',
         '{{ line.unit_price_incl_tax }}',
         '{{ line.quantity }}']);
 {% endfor %}