libnvdimm, region: hide persistence_domain when unknown

Similar to other region attributes, do not emit the persistence_domain
attribute if its contents are empty.

Fixes: 96c3a239 ("libnvdimm: expose platform persistence attr...")
Cc: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>