scylla_io_setup: fix data_file_directories check
scylla_io_setup: fix data_file_directories check

Use default data_file_directories if it's not assigned in scylla.yaml

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla