Invalid data

[tkurki]

There are several explicit cases where a sensor can indicate that it is otherwise operational but can not produce vali ddata. Example cases are a depth sounder that can not discern bottom from the echo data and a GNSS that has no satellite fix.

In these cases a gateway/server will typically receive a message indicating invalid data. It must send out a delta message where the value of the data item is null and serve the value as null in the REST api.