rbd: fix CLI output of `rbd group snap info` command

rbd: fix CLI output of `rbd group snap info` command

... when a group snapshot has no member images.

A group snapshot can be created with no member images. For such a group
snapshot, omit the 'image snap' and 'images' fields from the
unformatted CLI output of `rbd group snap info` command so as to not
confuse the user. In the librbd C/C++ data structures representing a
group snapshot with no member images, set the 'image_snap_name' data
member to an empty string.

Fixes: https://tracker.ceph.com/issues/67436
Signed-off-by: Ramana Raja <rraja@redhat.com>