In Part 6 while describing the OPC UA JSON encoding the spec is clear that structure fields with a null value are omitted from the JSON object, but for all of the built-in types, which are defined in the spec before the encoding for structures, no specific recommendation is given and thus it's not clear what to do.

That is, given e.g. a DiagnosticInfo:

var diagnosticInfo = new DiagnosticInfo(
0,
1,
2,
3,
"foo",
null,
null
);

Is the correct encoding:

{"SymbolicId":1,"NamespaceUri":0,"Locale":2,"LocalizedText":3,"AdditionalInfo":"foo","InnerStatusCode":null,"InnerDiagnosticInfo":null}

or:

{"SymbolicId":1,"NamespaceUri":0,"Locale":2,"LocalizedText":3,"AdditionalInfo":"foo"}

Speaking with Randy he said the recommendation is to omit the fields. The spec should explicitly mention this, either in each "struct-like" built-in type, or as a general recommendation before they are defined.