Folder_generateWorkflowReport: Make selection-related parameters optional.

Fixes test failures.

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/Folder_generateWorkflowReport.xml

@@ -124,7 +124,7 @@ return context.Folder_viewWorkflowReport()\n
         </item>
         <item>
             <key> <string>_params</string> </key>
-            <value> <string>use_selection, selection_name</string> </value>
+            <value> <string>use_selection=False, selection_name=None</string> </value>
         </item>
         <item>
             <key> <string>_proxy_roles</string> </key>