[BUG] possible NPE in SnapshotsService.java

Describe the bug

Hello.

We've analyzed Opensearch sources with SAST and it pointed out possible NPE in https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/snapshots/SnapshotsService.java#L3410

                final IndexRoutingTable indexRoutingTable = routingTable.index(indexName);
                for (int i = 0; i < indexMetadata.getNumberOfShards(); i++) {
                    final ShardId shardId = indexRoutingTable.shard(i).shardId();
                    final String shardRepoGeneration;

                    final String inFlightGeneration = inFlightShardStates.generationForShard(index, shardId.id(), shardGenerations);
                    if (inFlightGeneration == null && isNewIndex) {
                        assert shardGenerations.getShardGen(index, shardId.getId()) == null : "Found shard generation for new index ["
                            + index
                            + "]";
                        shardRepoGeneration = ShardGenerations.NEW_SHARD_GEN;
                    } else {
                        shardRepoGeneration = inFlightGeneration;
                    }
                    final ShardSnapshotStatus shardSnapshotStatus;
                    if (indexRoutingTable == null) {
                        shardSnapshotStatus = new ShardSnapshotStatus(

If indexRoutingTable is null, then shards processing will throw NPE.
I didn't get this peace of code, so even can't propose PR, it'll be nice if someone more experienced will take a look

Related component

No response

To Reproduce

Didn't catch it in action, just static analysis of the code

Expected behavior

No NPE if indexRoutingTable is null

Additional Details

Read server/src/main/java/org/opensearch/snapshots/SnapshotsService.java around line 3410 and trace how indexRoutingTable is obtained before shard processing. Confirm whether the reported null state is reachable, then locate or add a focused snapshot-service regression test; the work is done when that state no longer causes an NPE.