Fix failing hibernate plugin test

The test included here is roughly the test from https://github.com/grails-plugins/grails-hibernate-plugin/blob/b80ddfb7cd3c1a80139caa1af49b7c89e3111ea2/test/unit/org/codehaus/groovy/grails/orm/hibernate/binding/AssociationDataBindingTests.groovy#L182
improve the handling of binding null values