fix(core): A runtime NullPointerException when extractValues is called with null property

[humberthardy]

Fix a runtime NullPointerException when extractValues is called with null property.

How to Reproduce

(this is a simplified version of the code, I hope it works):

Define a Gorm model like this

class Category() {
  int id 
  String name
  static mappedBy = [children: "children"]

   List <Category> children {
     def c = CategoryRelation.createCriteria()
     def ids = c.list( eq (b, id) )
     return this.createCriteria().list(in ('id', ids)
  }

}

class CategoryRelation() {
a Category,
b Category
  static mapping = {
    id composite: [a, b]
  }
}

def parent = new Category(name: "parent").save()
def child = new Category(name: "Child").save()
def relation = new CategoryRelation(a: parent, b: child).save()

>>> parent as JSON

In this case, it will call DomainClassMarshaller.asShortObject(_, _, referencedIdProperty *<= null*, CategoryRelation)