lsblk: ignore only loopdevs without backing file

lsblk: ignore only loopdevs without backing file

* do not ignore all empty devices, we need more smart solution

* ignore only loop devices without backing file, for example:
 # touch img
 # losetup -f img
 losetup: img: Warning: file is smaller than 512 bytes; the loop device may be useless or invisible for system tools.

 - old version display nothing
 - new version:

 # lsblk /dev/loop0
 NAME  MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
 loop0   7:0    0   0B  0 loop

Addresses: https://github.com/karelzak/util-linux/issues/1118
Signed-off-by: Karel Zak <kzak@redhat.com>