Lucene doesn't support sorting missing value as anything else than "first" or "last" for the STRING and STRING_VAL types. Currently we just let Lucene throw an exception whenever the user calls onMissingValue().use( ... ) on such type.
We should at the very least throw our own exception like we used to in Search 5 (see org.hibernate.search.query.dsl.sort.impl.SortFieldStates#processMissingValue).
Ideally we should support it instead of failing. It would require implementing our own sorting mechanism for strings, though; not sure it's worth it.