mon/PGMap: return empty stats if pool is not in sum

Greg was right!

When a pool is created, the PGs are not added to the PGMap until the *next* 
proposal.  Weaken the assert here and return empty stats for non-existent
(new) pools so that a pool create + tier add sequence does not crash.

Signed-off-by: Sage Weil <sage@inktank.com>