ListBox: Fix 6fd21826 backward-compatibility code (#20161014-741678).

Before fixing this bug, 'listbox_previousPage:method' input button was
submitted but with no value (''). However, this was not handled by the
backward-compatibility code and thus next/previous page buttons would
not work properly if erp5_xhtml_style is not updated.

product/ERP5Form/Tool/SelectionTool.py

@@ -1826,9 +1826,8 @@ for property_id in candidate_method_id_list:
     """
     portal_selection = getToolByName(self, 'portal_selections')
     request = self.REQUEST
-    try:
-      listbox_id = request.form['listbox_%s' % wrapper_property_id]
-    except KeyError:
+    listbox_id = request.form.get('listbox_%s' % wrapper_property_id, None)
+    if not listbox_id:
       # Backward-compatibility: Should be removed as soon as
       # createFolderMixInPageSelectionMethod has been removed
       warnings.warn(