ceph-objectstore-tool: update-mon-db: do not fail if incmap is missing

ceph-objectstore-tool: update-mon-db: do not fail if incmap is missing

there is chance that we could use an OSD which does not have incmap of a
certain epoch for rebuilding the monstore. and since OSD does not read
and store the incmap if the MOSDMap message already has the fullmap of
that fullmap, and if an OSD does not have previous fullmap, monitor
will just send it the fullmao. so it's not unusual that an OSD has
a fullmap of some epoch without corresponding incmap.

Fixes: https://tracker.ceph.com/issues/41177
Signed-off-by: Kefu Chai <kchai@redhat.com>