When an empty pubsub node is being retrieved, the CachingPubsubPersistewnceProvider attempts to cache its content. As the content is null, a nullpointerexception occurs.
Ideally, we'd store an Optional (to avoid further database calls that won't have a result), but pragmatically, simply skipping the cache would probably be good enough.
I don't think that this bug exists in code that was in a release (that's not a beta-release), as this involves code that was created for the upcoming 4.6.0 release.
Potential workaround: use DefaultPubsubPersistenceProvider directly, instead of CachingPubsubPersistenceProvider (that's backed by DefaultPubsubPersistenceProvider).