ublk: remove unnecessary NULL check in ublk_rq_has_data()

bio_has_data() allows a NULL bio so the NULL check in
ublk_rq_has_data() is unnecessary.

Signed-off-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230207070839.370817-2-ZiyangZhang@linux.alibaba.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>