Do not allow empty string for media descriptor. For the sake of clearness NULL should be used for a missing value. This avoids extra checks e.g. for generating LAF exports. The CHECK constraints in the database could be more elaborate.

{
    "land.ortszuordnung": [{
        "id": 1000,
        "probe_id": 1000,
        "ort_id": 23,
        "ortszuordnung_typ": "E",
        "ortszusatztext": "Test",
        "letzte_aenderung": "2012-05-07 09:00:00"
    }]
}