OSDMonitor.h: use !reporters.empty() instead of size()

OSDMonitor.h: use !reporters.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().

Using empty() in this case makes the code more logical.

warning from cppcheck was:

[src/mon/OSDMonitor.h:59]: (performance) Possible inefficient
  checking for 'reporters' emptiness.

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