Fix usage of Session::unserialize()
Fix usage of Session::unserialize()

This function should be checking new value for emptiness, but it was checking
the database value instead. Additionally, at this point, the database value
would always be `NULL`. Changed to use the correct variable.

Picked from d2e292df21

michael-e/symphony-2,jensscherbl/symphony,bauhouse/sym-todo-list,symphonycms/symphony-2,jensscherbl/symphony,symphonycms/symphony-2,michael-e/symphony-2,bauhouse/sym-todo-list