erp5_auto_logout: Fix /logout url access when not being logged in.

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_auto_logout/logout.py

 from AccessControl import getSecurityManager
 portal = context.getPortalObject()
-portal.portal_sessions.manage_delObjects(
+username = getSecurityManager().getUser().getUserName()
+if username is not None:
+  portal.portal_sessions.manage_delObjects(
     portal.Base_getAutoLogoutSessionKey(
-    username=getSecurityManager().getUser().getUserName(),
+      username=username,
+    )
   )
-)
 REQUEST = portal.REQUEST
 if REQUEST.has_key('portal_skin'):
   portal.portal_skins.clearSkinCookie()