We currently have 76 rows where there is nothing useful at all:
musicbrainz_db_20110516=> select count(*) from release_label where label is null and catalog_number is null;
In review at https://github.com/metabrainz/musicbrainz-server/pull/38
Apparently the upgrade script from this was not run, and so CreateConstraints was failing on our full-import tests. I've now run it, so as of the next dump those should stop being a problem.