[fix] to_xhtml: avoid stupid runtime errors like
Can't make an xml with {args : list(Xml.attribute); content : list(xml(xhtml_specific_attributes ,xhtml_specific_extensions)); namespace : string; specific_attributes : option(xhtml_specific_attributes); tag : string; xmlns : list(Xml.binding)} / {content_unsafe : string} / {fragment : list(xml(xhtml_specific_attributes ,xhtml_specific_extensions))} / {text : string} / {xml_dialect : option(xhtml_specific_extensions)}}