HDFS-7645 changed rollingUpgradeInfo to still return an RUInfo after finalization, so the DNs can differentiate between rollback and a finalization. However, this breaks compatibility for the user facing APIs, which always expect a null after finalization. Let's fix this and edify it in unit tests.
As an additional improvement, isFinalized and isStarted are part of the Java API, but not in the JMX output of RollingUpgradeInfo. It'd be nice to expose these booleans so JMX users don't need to do the != 0 check that possibly exposes our implementation details.