cls/rbd/cls_rbd.cc: use !empty() instead of 'size() > 0'

cls/rbd/cls_rbd.cc: use !empty() instead of 'size() > 0'

Use empty() since it should be prefered as it has, following the
standard, a constant time complexity regardless of the containter
type. The same is not guaranteed for size().

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>