mgr/DaemonServer: osd ok-to-stop: return json when there are unknown PGs

mgr/DaemonServer: osd ok-to-stop: return json when there are unknown PGs

In 791952cc01201010f298033003ba52374cc0159f we switched to return JSON
both on success and fail to describe which PGs are affected or are blocking
the ability to stop/restart OSDs.  Do the same for the case where
some PG states are unknown (i.e., just after a mgr restart) so that
the cephadm upgrade process can unconditionally expect a JSON result.

Signed-off-by: Sage Weil <sage@newdream.net>
(cherry picked from commit 2cce16537c9f9e92bf0a8fce746d0364cbb2d15d)