DatabaseFactory.findCorrectDatabaseImplementation() has a special case to use UnsupportedDatabase if all else fails. That code fails to pass the DatabaseConnection to the new UnsupportedDatabase object, thus causing NPEs later on.
This is the issue that was discussed in http://forum.liquibase.org/topic/problem-with-sql-tag-on-unsupported-database; a fix was committed for 3.0, but not for the 2.0.x branch.
The attached patch backports the fix from 3.0.
CentOS 6.3, Java 1.6.0_24