Monitor.cc: use empty() instead of size()

Monitor.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 cppcheck was:
[src/mon/Monitor.cc:487]: (performance) Possible inefficient
  checking for 'initial_members' emptiness.
[src/mon/Monitor.cc:1361]: (performance) Possible inefficient
  checking for 'timecheck_skews' emptiness.
[src/mon/Monitor.cc:2302]: (performance) Possible inefficient
  checking for 'timecheck_waiting' emptiness.
[src/mon/Monitor.cc:2547]: (performance) Possible inefficient
  checking for 'timecheck_waiting' emptiness.

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