data sources with null value

[Asw1n]

The plug in configuration page shows a thumbs-up icon when a data source is available. In some cases a data source might be available but will always have a null value. My GPS for example generates an RMC message, but it does not report magnetic variation in this message. SignalK will import this message and create a navigation.magneticVariation with a null value.
The thumbs-up icon suggests a calculation can be made, but the calculation (true heading in my example) will never be made. There is also no indication in the log as to why the calculation has not been made. This makes it hard to find tho problem.

It would be nice if either:

• the plug in reports null data in the data log.
• there would be an alternative icon in the plugin to indicate consistent null values.