[ML] Fix possible race condition starting datafeed

Datafeeds being closed while starting could result in and NPE. This was
handled as any other failure, masking out the NPE. However, this
conflicts with the changes in elastic#50886.

Related to elastic#50886 and elastic#51302

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/datafeed/DatafeedManager.java

@@ -520,7 +520,12 @@ private void runTask(TransportStartDatafeedAction.DatafeedTask task) {
             // a context with sufficient permissions would coincidentally be in force in some single node
             // tests, leading to bugs not caught in CI due to many tests running in single node test clusters.
             try (ThreadContext.StoredContext ignore = threadPool.getThreadContext().stashContext()) {
-                innerRun(runningDatafeedsOnThisNode.get(task.getAllocationId()), task.getDatafeedStartTime(), task.getEndTime());
+                Holder holder = runningDatafeedsOnThisNode.get(task.getAllocationId());
+                if (holder != null) {
+                    innerRun(holder, task.getDatafeedStartTime(), task.getEndTime());
+                } else {
+                    logger.warn("Datafeed [{}] was closed while being opened", task.getDatafeedId());
+                }
             }
         }