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_m": [{
        "id":1000,
        "erzeuger":"06010",
        "datum":"2012-05-08 11:00:00",
        "text":"Testkommentar",
        "messungs_id":1200
    }]
}