I'm seeing quite a few of these in prod:

```
java.lang.NullPointerException: Cannot invoke "org.elasticsearch.xpack.core.transform.transforms.TransformState.getTaskState()" because "transformState" is null
    at org.elasticsearch.xpack.transform.TransformUsageTransportAction.masterOperation(TransformUsageTransportAction.java:106) ~[?:?]
    at org.elasticsearch.xpack.transform.TransformUsageTransportAction.masterOperation(TransformUsageTransportAction.java:55) ~[?:?]
    at org.elasticsearch.action.support.master.TransportMasterNodeAction.executeMasterOperation(TransportMasterNodeAction.java:124) ~[elasticsearch-8.9.0.jar:?]
    at org.elasticsearch.action.support.master.TransportMasterNodeAction$AsyncSingleAction.lambda$doStart$3(TransportMasterNodeAction.java:235) ~[elasticsearch-8.9.0.jar:?]
    at org.elasticsearch.action.ActionRunnable$3.doRun(ActionRunnable.java:72) ~[elasticsearch-8.9.0.jar:?]
    at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:983) ~[elasticsearch-8.9.0.jar:?]
    at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26) ~[elasticsearch-8.9.0.jar:?]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[?:?]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[?:?]
    at java.lang.Thread.run(Thread.java:1623) ~[?:?]
```

`org.elasticsearch.persistent.PersistentTasksCustomMetadata.PersistentTask#getState` is declared `@Nullable` but this case is not handled properly by the usage action.
x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/action/TransportStartTransformAction.java
@Override
        public boolean test(PersistentTasksCustomMetadata.PersistentTask<?> persistentTask) {
            if (persistentTask == null) {
                return false;
            }
            PersistentTasksCustomMetadata.Assignment assignment = persistentTask.getAssignment();
            if (assignment != null
                && assignment.equals(PersistentTasksCustomMetadata.INITIAL_ASSIGNMENT) == false
                && assignment.isAssigned() == false) {
                
                
                exception = new ElasticsearchStatusException(
                    "Could not start transform, allocation explanation [" + assignment.getExplanation() + "]",
                    RestStatus.TOO_MANY_REQUESTS
                );
                return true;
            }
            
            return assignment != null && assignment.isAssigned() && isNotStopped(persistentTask);
        }
        
        
        
        private static boolean isNotStopped(PersistentTasksCustomMetadata.PersistentTask<?> task) {
            TransformState state = (TransformState) task.getState();
            return state != null && state.getTaskState().equals(TransformTaskState.STOPPED) == false;
        }
    }
}