Null fields in JSON deserialization (lang::json::IO)

[tvdstorm]

Apparently, null fields in JSON are ignored (or trigger exceptions):

rascal>import lang::json::IO;
rascal>parseJSON(#value, "{\"a\": null}");
value: "object"()
rascal>parseJSON(#value, "{\"a\": 2}");
value: "object"(
 a=2)

With an explicit type, it triggers an exception:

rascal>data Foo = foo(int a=-1);
ok
rascal>parseJSON(#Foo, "{\"a\": 2}");
Foo: foo(a=2)
rascal>parseJSON(#Foo, "{\"a\": null}");
|std:///lang/json/IO.rsc|(2599,303,<49,0>,<52,144>): Java("IllegalStateException","Expected a name but was NULL at line 1 column 11 path $.a")
        at com.google.gson.stream.JsonReader.nextName(|unknown:///JsonReader.java|(0,0,<800,0>,<800,0>))
        at org.rascalmpl.library.lang.json.internal.JsonValueReader$2.visitAbstractData(|unknown:///JsonValueReader.java|(0,0,<517,0>,<517,0>))
        at org.rascalmpl.library.lang.json.internal.JsonValueReader$2.visitAbstractData(|unknown:///JsonValueReader.java|(0,0,<116,0>,<116,0>))
        at io.usethesource.vallang.type.AbstractDataType.accept(|unknown:///AbstractDataType.java|(0,0,<422,0>,<422,0>))
        at org.rascalmpl.library.lang.json.internal.JsonValueReader.read(|unknown:///JsonValueReader.java|(0,0,<116,0>,<116,0>))
        at org.rascalmpl.library.lang.json.IO.parseJSON(|unknown:///IO.java|(0,0,<130,0>,<130,0>))
        at parseJSON(|std:///lang/json/IO.rsc|(2895,5,<52,137>,<52,142>)ok