web: also set empty criterion as invalid on Web Site

For same reasons as on Web Sections: clicking the Contents tab search
for all documents

bt5/erp5_web/SkinTemplateItem/portal_skins/erp5_web/WebSite_init.py

@@ -10,3 +10,4 @@ localizer = context.getPortalObject().Localizer
 context.setAvailableLanguageSet(localizer.get_supported_languages())
 context.setDefaultAvailableLanguage(localizer.get_selected_language())
 context.setStaticLanguageSelection(1)
+context.setEmptyCriterionValid(False)