[FIX] web_editor: do not break labels' texts into <p/>

When editing the content of a <label/> which has XML branding (first
DOM element which is editable in its hierarchy), a <p/> element was
added inside. This behavior is there to automatically add <p/> elements
in empty editable <div/> / <section/> elements on edition. This should
however not apply on <label/> elements.