mon/OSDMonitor: fix base case for 7fb3804fb workaround

After cluster creation, we have no full map stored and first_committed ==
1.  In that case, there is no need for a full map, since we can get there
from OSDMap() and the incrementals.

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao@inktank.com>
(cherry picked from commit e807770784175b05130bba938113fdbf874f152e)