default_package_show_schema resources can cause 500 error.

The current [default_show_package_schema](ckan/blob/master/ckan/logic/schema.py#L219) adds some fields so they don't get stripped if they are missing to the resource subschema, but at the moment the empty validators inject a missing object in their place. But Missing is not jsonable and causes 500s in the [package](ckan/controllers/package.py#L510), [related](ckan/controllers/related.py#L120) and in the api controller