mkfs: fix parsing of value-less -d/-l concurrency cli option

mkfs: fix parsing of value-less -d/-l concurrency cli option

It's supposed to be possible to specify the -d concurrency option with
no value in order to get mkfs calculate the agcount from the number of
CPUs.  Unfortunately I forgot to handle that case (optarg is null) so
mkfs crashes instead.  Fix that.

Fixes: 9338bc8b1bf073 ("mkfs: allow sizing allocation groups for concurrency")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>