libblkid: don't return error on empty files

libblkid: don't return error on empty files

Currently, the library does not allow to initialize blkid_probe if the
file (or block device) is empty. The empty file is reported as an
error. That's wrong. The empty file should be interpreted as a file
without any FS or PT. It means that

	blkid_do_{probe,safeprobe,fullprobe}()

have to return 1 ("nothing").

Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Karel Zak <kzak@redhat.com>