AbstractReferenceUnmarshaller cannot handle null values for references

For my custom converter, I would sometimes like to return null when unmarshalling. If there is a reference to it further down in the xml, I would expect that to be set to null as well. However, AbstractReferenceUnmarshaller.convert() will assume a null value means the reference key was not in the map, and throw an exception. Instead it should use the containsKey() method to check whether or not the key exists in the map.