The size of the data to store is checked.

When storing a session, it saves, even if data size is 0. 
When tested in my environment, the session of size 0 was also saved.
(Mayby, my application problem...)

I think, library clean up garbage session from DynamoDB, when session expired.
But, Excessive storing is cost.
So, I corrected it.

Please, marge source.

Thanks.