mgr/dashboard: Use the default pg num instead of "1" when creating a pool with autoscaling enabled
This is a refinement/followup to #38227 : currently, the dashboards sets the
pg_num value to 1 if PG autoscaling is enabled. During today's CDM call, Sage suggested that the dashboard should just leave the value empty so that the cluster's default PG num value would be used.
Alternatively, the dashboard could obtain the cluster's default value (is there a setting for this?) and use that value instead of defaulting to 1.