Adjust filesystem check for new blkid behaviour

blkid used to return with an exit code of 2 if 'blkid -s TYPE -o
value' was executed on an unformated device.  Nowadays blkid
returns with exit code 0 even if there's no file system on the
according device.

Related to #772846 in Debian.

Closes: #772849
Release new version 0.68