Fix NullPointerException in shard snapshot code (#111524)

Fixing a NullPointerException by instead throwing
ShardNotFoundException when the IndexShard is null/gone.

Author: DiannaHohensee

docs/changelog/111524.yaml

@@ -0,0 +1,5 @@
+pr: 111524
+summary: Fix `NullPointerException` in shard snapshot code
+area: Snapshot/Restore
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/snapshots/SnapshotShardsService.java

@@ -360,7 +360,7 @@ private void snapshot(
         ActionListener<ShardSnapshotResult> listener
     ) {
         try {
-            final IndexShard indexShard = indicesService.indexServiceSafe(shardId.getIndex()).getShardOrNull(shardId.id());
+            final IndexShard indexShard = indicesService.indexServiceSafe(shardId.getIndex()).getShard(shardId.id());
             if (indexShard.routingEntry().primary() == false) {
                 throw new IndexShardSnapshotFailedException(shardId, "snapshot should be performed only on primary");
             }