There isn't a built-in precondition that will check that, but you could
either use the <sqlCheck> precondition and query the metadata
tables in the check, or write your own extension that can check
nullability without needing a constraint name.
liquibase.org/extensions has some documentation on writing an
extension. The advantage with an extension is that then you don't need
to keep writing the SQL in each precondition and can use the liquibase
snapshot java APIs to get at the metadata without figuring out the SQL.
If you wanted to send a pull request for a new precondition in the
main liquibase code, I would certainly look at incorporating it as well.
nathan