Fixed possible 0257_fix_has_link_attribute.py database migration failure, which would cause errors during the upgrade process.
Note: Although this is marked as a “pre-release” on GitHub to avoid displacing 4.5 as the latest release, this is really a full release from the 2.1.x branch. We recommend that most users install or upgrade to the latest release, but users who need to stay on the 2.1.x branch can upgrade to 2.1.8.