When the data is null, it should not throw an Exception

Not only race condition may lead to getting data from redis is "null",  but no data session can cause the "NULL_SESSION" as well. When access some wrong page with 404 HTTP Code, A Null Session is made in the redis. When trying to get the session again,  an IllegalStateException is thrown. 

The better way , from my perspective, is to create a new session without data. Just like some old versions for the plugin. 

Anyway, thank you for the beneficial session manager, which helps me a lot.