Fix shared_file_storage_dir on upgrades

If the cluster was upgraded from 2.4 or earlier, this key won't exist
(it's only set to a correct value on cluster init), so we need to
properly set it to a null string (disabled).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>