[web/form] Relax condition for non-translating field names we should test true condition instead of None, so if one raise a ValidationError using an empty string as 'no-field' marker instead of None, we don't get the PO file header that gettext likes to return when one attempt to translate an empty string.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)