null_blk: Fix REQ_OP_ZONE_CLOSE handling

In order to match ZBC defined behavior, closing an empty zone must
result in the "empty" zone condition instead of the "closed" condition.

Fixes: da644b2c ("null_blk: add zone open, close, and finish support")
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>