Self-closing markup tags should be disallowed or handled correctly

> The XML has a self-closing `` tag, which is technically allowed by the schema. I believe it’s not valid HTML, though, which may be why some part of the Hugo pipeline turns it into a ``. I guess we could (i) check if the schema can be written to disallow empty markup elements, and/or (ii) replace these with something valid in our `MarkupText.as_html()`.

_Originally posted by @mbollmann in [#6407](https://github.com/acl-org/acl-anthology/issues/6407#issuecomment-3507894193)_

Start by reading the schema handling the self-closing element and the MarkupText.as_html() entry point, then trace where the Hugo pipeline changes it. Done means the project consistently either rejects empty markup elements or renders them as valid HTML, with the chosen behavior verified in the relevant checks.

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