src/osd/OSD.h: use empty() instead of size()

src/osd/OSD.h: use empty() instead of size()

Fix warning for usage 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:
[osd/OSD.h:265]: (performance) Possible inefficient checking for
   'last_scrub_pg' emptiness.
[osd/OSD.h:274]: (performance) Possible inefficient checking for
   'last_scrub_pg' emptiness.

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