null_blk: Fix zone write handling

null_zone_write() only allows writing empty and implicitly opened zones.
Writing to closed and explicitly opened zones must also be allowed and
the zone condition must be transitioned to implicit open if the zone
is not explicitly opened already.

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>