turns out ElementTree has a hard time XML-serializing ``None`` (who'd have guessed...), which is what it gets told to do when concat_xml is called with an empty list of files. Return an empty string and no timestamp (should probably be one there at some point, but...) and fix the QWeb loader to not do anything when it gets an empty template file. bzr revid: xmo@openerp.com-20120113150641-6i3ot1jg7r3kpw3d