NPE from marshalling an IDREF that point to a missing ID attribute value

NPE is thrown prematurely in case of an id attribute being null instead of a more appropriate validation event (Object "..." is found in an IDREF property but this object doesnt have an ID.). In order to fix this see AttributeProperty.serializeAttributes method that deals with the null correctly.

It seems that other people have had to deal this problem before ([http://stackoverflow.com/questions/10705532/in-jaxb-marshalling-how-to-identify-which-child-element-caused-jaxb-marshallin](http://stackoverflow.com/questions/10705532/in-jaxb-marshalling-how-to-identify-which-child-element-caused-jaxb-marshallin)). I guess that pinpointing of the cause can be quite problematic with the current behavior.

* * StackTrace * *
AttributeProperty.getIdValue(BeanT) line: 128
ClassBeanInfoImpl.getId(BeanT, XMLSerializer) line: 322
TransducedAccessor$IDREFTransducedAccessorImpl.print(BeanT) line: 290
TransducedAccessor$IDREFTransducedAccessorImpl(DefaultTransducedAccessor).writeLeafElement(XMLSerializer, Name, T, String) line: 69
SingleElementLeafProperty.serializeBody(BeanT, XMLSerializer, Object) line: 130
ClassBeanInfoImpl.serializeBody(BeanT, XMLSerializer) line: 361
XMLSerializer.childAsXsiType(Object, String, JaxBeanInfo, boolean) line: 696