ReferenceIdSchemaNode needs to serialize None

[tseaver]

The _reference_property implementation [returns None]](https://github.com/Pylons/substanced/blob/e8b1038053a4f5dc601c323add7cf57b533ab27f/substanced/objectmap/__init__.py#L789) if the requested reference does not exist, but that value is invalid for the schema type (colander.Int). It needs to override serialize`, e.g.:

    def serialize(self, appstruct):
        if appstruct is None:
            return colander.null
        return super().serialize(appstruct)