sfdisk: support empty label use-case

By default sfdisk creates partition table when a first partition is
specified, otherwise the device is not modified. This force users to
create at least one partition.

This commit allows to create empty label without partitions if "label:
<name>" header line is specified by script.

The commit also modifies "New situation:" output to list label name
and label identifier.

Addresses: https://github.com/karelzak/util-linux/issues/374
Signed-off-by: Karel Zak <kzak@redhat.com>