ceph_rest_api.py: don't fail if no up OSDs found on startup

allow find_up_osd() to return None, and have caller check for it

Fixes: #7463
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #1261 from ceph/wip-7463

ceph_rest_api.py: don't fail if no up OSDs found on startup

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>