Merge pull request #3495 from XinzeChi/wip-size-empty

cleanup: replace some size() or length() with !empty()

Reviewed-by: Loic Dachary <ldachary@redhat.com>

diff --cc src/mds/MDS.cc
Simple merge

diff --cc src/mon/OSDMonitor.cc
Simple merge

diff --cc src/osd/OSD.cc
Simple merge

diff --cc src/osd/ReplicatedPG.cc
Simple merge