Fix NPE in DomainClassMarshaller

I found an NPE in DomainClassMarshaller which is caused by changes in JSONWriter in commit 0aa1ec82ba236cac3f8cf6d4c139b67a26a19b3c.  A call to writer.value(null) causes JSONWriter.value(Number) to be called, which in turn throws the NPE. In that commit method valueNull() was introduced, so I use it instead.

I'm not sure if line 160 (json.value(null)) must be changed, too.
Merge pull request #429 from dellermann/patch-1

Fix NPE in DomainClassMarshaller
GRAILS-10799 related: Fix NPEs in JSONWriter