erp5_web: getAvailableLanguageList may return nothing or []

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web/WebSection_checkDefaultPageModificationDateConsistency.py

@@ -8,7 +8,7 @@ if context.getAggregate():
   max_content_modification_date = context.getModificationDate()
   for default_page_reference in context.getAggregateReferenceList():
     if default_page_reference:
-      for language in (None,) + context.getAvailableLanguageList():
+      for language in (None,) + (context.getAvailableLanguageList() or ()):
         default_page = getDocumentValue(
             default_page_reference,
             language=language,