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
GRAILS-10799 related: Test null handling in JSONWriter
Fixed for GRAILS-9778 - Cygwin shows empty path
fix bug that prevented overriding the transaction manager via a custom setter in @Transactional
GRAILS-10853 - improve binding to Set using indexes
GRAILS-10853 - ignore 2 tests
GRAILS-10853 - fix broken tests to be consistent with actual expectations
remove println
GRAILS-10865 - improve binding to inherited typed collections
fix test which should not be relying on order in a Set
GRAILS-10866 - fix compile error for controller exception handlers for Groovy exceptions
GRAILS-10838 - introduce TypedStructuredBindingEditor
update version to 2.3.4
Merge branch '2.3.x' of github.com:grails/grails-core into 2.3.x
GRAILS-10868 - improve binding XML to persistent collections
GRAILS-10868 - improve binding XML to persistent collections

This addresses a problem where XML is being bound to a persistent collection and the XML contains only 1 child element to add to the collection
remove unused import statement
GRAILS-10871 - improve non domain class validation

The validator should ignore properties which already have binding errors associated with them.