Paxos.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().

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

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
FileStore.cc: use if(!empty()) instead of if(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().

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