PG: don't query unfound on empty pgs

When the replica responds, it responds with a notify
rather than a log, which the primary then ignores since
it is already in the peer_info map.  Rather than fix that
we'll simply not send queries to peers we already know to
have no unfound objects.

Fixes: #6910
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
(cherry picked from commit 838b6c8387087543ce50837277f7f6b52ae87d00)