Ensure task metadata not null in follow test (elastic#37993)

This commit fixes a potential race in the IndexFollowingIT. Currently it
is possible that we fetch the task metadata, it is null, and that throws
a null pointer exception. Assertbusy does not catch null pointer
exceptions. This commit assertions that the metadata is not null.

Author: Tim-Brooks

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/IndexFollowingIT.java

@@ -983,6 +983,7 @@ private CheckedRunnable<Exception> assertTask(final int numberOfPrimaryShards, f
         return () -> {
             final ClusterState clusterState = followerClient().admin().cluster().prepareState().get().getState();
             final PersistentTasksCustomMetaData taskMetadata = clusterState.getMetaData().custom(PersistentTasksCustomMetaData.TYPE);
+            assertNotNull(taskMetadata);
 
             ListTasksRequest listTasksRequest = new ListTasksRequest();
             listTasksRequest.setDetailed(true);