OSD::handle_pg_query: on dne pg, send lb=hobject_t() if deleting

We will set lb=hobject_t() if we resurrect the pg.  In that case,
we need to have sent that to the primary before hand.  If we
finish the removal before the pg is recreated, we'll just end
up backfilling it, which is ok since the pg doesn't exist anyway.

Fixes: #7740
Signed-off-by: Samuel Just <sam.just@inktank.com>