Metric with null value (IsNull) do not survice decoding

[simmarn]

How to reproduce.

1. Create one EdgeNode and one Application
2. Send NDATA or DDATA from EdgeNode with metric containing a null value. For example a string metric with value null. At inspection of the protobuf coded payload data IsNull = true, which is correct
3. At reception in Application the metric value is string.Empty not null and IsNull = false. THIS IS THE FAULT.

Please release a fixed version as our interface depends on null values being passed correctly. We would also like to get in touch to discuss how to support your work with SparkplugNet.

[simmarn]

I created a PR: #123