Fix Segment Replication stats throwing NPE (opensearch-project#14580)

Signed-off-by: Rampreeth Ethiraj <ethirajrampreeth@gmail.com>

Author: rampreeth

server/src/main/java/org/opensearch/index/seqno/ReplicationTracker.java

@@ -1254,8 +1254,9 @@ public ReplicationCheckpoint getLatestReplicationCheckpoint() {
 
     // skip any shard that is a relocating primary or search only replica (not tracked by primary)
     private boolean shouldSkipReplicationTimer(String allocationId) {
-        Optional<ShardRouting> shardRouting = routingTable.shards()
+        Optional<ShardRouting> shardRouting = routingTable.assignedShards()
             .stream()
+            .filter(routing -> Objects.nonNull(routing.allocationId()))
             .filter(routing -> routing.allocationId().getId().equals(allocationId))
             .findAny();
         return shardRouting.isPresent() && (shardRouting.get().primary() || shardRouting.get().isSearchOnly());