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.kommentar_p": [{
        "id": 1000,
        "erzeuger": "06010",
        "datum": "2012-05-11 12:01:00",
        "text": "Testkommentar",
        "probe_id": 1000
    }]
}