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.

{
    "messungsId": 1200,
    "messgroesseId": 242,
    "messwert":1,
    "messwertNwg":"",
    "messfehler":1,
    "nwgZuMesswert":3,
    "mehId":207,
    "grenzwertueberschreitung":false
}