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.

{
    "probeId":"1000",
    "mmtId":"A4",
    "nebenprobenNr":"10R1",
    "messdauer":10,
    "fertig":false,
    "geplant":true,
    "messzeitpunkt":"2015-02-09T10:58:36"
}