I’m not completely clear what you mean by "liquibase update against the new empty database.
If that is not the problem you are having, I could interpret your question as “The databasechangelog.xml file that was created by the generateChangeLog command was empty.” If that is what is happening, the most likely issue is that liquibase is either not connecting to the database you expect, or the database is indeed empty, or there are some sort of permissions problems and the credentials you are using to connect to the database are insufficient to be able to query the system tables required.
Steve Donie
Principal Software Engineer
Datical, Inc. http://www.datical.com/