JSON writer generates malformed output when inner content has null fields

If you are serializing objects that contain objects and the inner object has no non-null fields or primitive types, the close bracket is not emitted.

This test case
===
public void testSimpleTypesCanBeRepresentedAsJsonWithNullMembers() {

String expected = (
"{'innerMessage': {\n" +
" 'greeting': 'hello',\n" +
" 'innerMessage':

\n" +
"}}").replace('\'', '"');

XStream xs = new XStream(new JsonHierarchicalStreamDriver());

xs.alias("innerMessage", Msg.class);

Msg message = new Msg("hello");

Msg message2 = new Msg(null);

message.innerMessage = message2;

String xs_sgtr = xs.toXML(message);
assertEquals(expected, xs_sgtr);
}

public static class Msg
{
String greeting;
Msg innerMessage;
public Msg(String greeting)

}
===
Should produce
===
{"innerMessage": {
"greeting": "hello",
"innerMessage": {
}
}}
===
But instead produces
===
{"innerMessage": {
"greeting": "hello",
"innerMessage":

===