Document model migration scripts leave documents that were never published in bad state

Running the HardHandleUpdateVisitor and then the HardDocumentUpdateVisitor leaves documents that were never published (i.e. that don't have any versions) in a bad state: in that case the unpublished variant misses the mix:versionable mixin. This is because the HardHandleUpdateVisitor assumes there is always a version history for the document and when that is replayed, the unpublished variant is already migrated correctly.