Fix NPE in ReindexDataStreamTransportAction (elastic#123262)

In the multi-project branch, we're making some changes to persistent
tasks and those changes can cause the persistent tasks custom to still
be `null`. This resulted in an NPE here, so I'm fixing the check here.

x-pack/plugin/migrate/src/main/java/org/elasticsearch/xpack/migrate/action/ReindexDataStreamTransportAction.java

@@ -83,11 +83,7 @@ protected void doExecute(Task task, ReindexDataStreamRequest request, ActionList
             ClientHelper.getPersistableSafeSecurityHeaders(transportService.getThreadPool().getThreadContext(), clusterService.state())
         );
         String persistentTaskId = getPersistentTaskId(sourceDataStreamName);
-
-        PersistentTasksCustomMetadata persistentTasksCustomMetadata = clusterService.state()
-            .getMetadata()
-            .custom(PersistentTasksCustomMetadata.TYPE);
-        PersistentTasksCustomMetadata.PersistentTask<?> persistentTask = persistentTasksCustomMetadata.getTask(persistentTaskId);
+        final var persistentTask = PersistentTasksCustomMetadata.getTaskWithId(clusterService.state(), persistentTaskId);
 
         if (persistentTask == null) {
             startTask(listener, persistentTaskId, params);