Fix CheckPointReplicationSlots() with max_replication_slots == 0

Fix CheckPointReplicationSlots() with max_replication_slots == 0

ca307d5cec90 made CheckPointReplicationSlots() unconditionally call
ReplicationSlotsComputeRequiredLSN().  It causes an assertion trap when
max_replication_slots equals 0.  This commit makes
CheckPointReplicationSlots() call ReplicationSlotsComputeRequiredLSN() only
when at least one slot gets its last_saved_restart_lsn updated.  That avoids
an assert trap and also saves some cycles when no one slot has
last_saved_restart_lsn updated.

Based on ideas from Dilip Kumar <dilipbalaut@gmail.com> and
Hayato Kuroda <kuroda.hayato@fujitsu.com>.

Reported-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Discussion: https://postgr.es/m/OS0PR01MB5716BB506AF934376FF3A8BB947BA%40OS0PR01MB5716.jpnprd01.prod.outlook.com