I've found this
Given an empty instance of an element whose content model is not EMPTY (for example, an empty title or paragraph) do not use the minimized form (e.g. use <p> </p> and not <p />)
But it is in the "HTML Compatibility Guidelines" appendix, i.e., it is not a prohibition in the specification, but a recommendation in order to have XHTML documents render in HTML-only browsers. I don't think it applies to ebook readers and ebook formats.