client/Client.cc: use empty() instead of size()

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().

warning from cppchecker was:
[src/client/Client.cc:3649]: (performance) Possible inefficient
  checking for 'mds_sessions' emptiness.
[src/client/Client.cc:7489]: (performance) Possible inefficient
  checking for 'osds' emptiness.

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