I do recognize (1) there is a logical difference between "missing because absent" and "missing because unknown", (2) empty string is not an element from the domain of personal names.

In a sense, empty string is available as a marker, just as NULL is available as a marker, to satisfy either scenario. I think the possibility of three-valued logic applies in either scenario.

I'm not at all convinced empty (non-null) string is appropriate for many real world applications, just as empty (non-null) date is appropriate for similar scenarios.

Eg. Date of death: unknown - NULL; not dead yet - NULL