Clarify the null check for retention leases (elastic#114979)

`MetadataStateFormat.FORMAT.loadLatestState` can actually return null when the state directory hasn't been
initialized yet, so we have to keep the null check when loading retention leases during the initialization of the engine.

See elastic#39359

server/src/main/java/org/elasticsearch/gateway/MetadataStateFormat.java

@@ -24,6 +24,7 @@
 import org.elasticsearch.common.lucene.store.InputStreamIndexInput;
 import org.elasticsearch.common.xcontent.LoggingDeprecationHandler;
 import org.elasticsearch.core.IOUtils;
+import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.Tuple;
 import org.elasticsearch.transport.Transports;
 import org.elasticsearch.xcontent.NamedXContentRegistry;
@@ -485,6 +486,7 @@ public Tuple<T, Long> loadLatestStateWithGeneration(Logger logger, NamedXContent
      * @param dataLocations the data-locations to try.
      * @return the latest state or <code>null</code> if no state was found.
      */
+    @Nullable
     public T loadLatestState(Logger logger, NamedXContentRegistry namedXContentRegistry, Path... dataLocations) throws IOException {
         return loadLatestStateWithGeneration(logger, namedXContentRegistry, dataLocations).v1();
     }

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

@@ -22,7 +22,6 @@
 import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.common.util.Maps;
 import org.elasticsearch.core.SuppressForbidden;
-import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.gateway.WriteStateException;
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexVersions;
@@ -471,9 +470,9 @@ public RetentionLeases loadRetentionLeases(final Path path) throws IOException {
         return emptyIfNull(retentionLeases);
     }
 
-    @UpdateForV9(owner = UpdateForV9.Owner.DISTRIBUTED_INDEXING)
     private static RetentionLeases emptyIfNull(RetentionLeases retentionLeases) {
-        // we expect never to see a null in 8.x, so adjust this to throw an exception from v9 onwards.
+        // `MetadataStateFormat.FORMAT.loadLatestState` can actually return null when the state directory
+        // on a node hasn't been initialized yet
         return retentionLeases == null ? RetentionLeases.EMPTY : retentionLeases;
     }