We can now have NULL fields in database, however currently we have no way of representing them in the xml for backups. If we have <data></data> it gets read as an empty string by restore. We plan to use a special string NULL to indicate a value of NULL in the xml, and restore that value during restore using insert_record().
-Add special NULL string to "full_tag()"
-Recover special NULL string from "backup_todb()"