[ML] Having no datafeed tasks can happen.

Original commit: elastic/x-pack-elasticsearch@b752ce4f0f

plugin/src/main/java/org/elasticsearch/xpack/ml/action/CloseJobAction.java

@@ -391,8 +391,7 @@ public class CloseJobAction extends Action<CloseJobAction.Request, CloseJobActio
                 } else {
                     // This can happen we the actual task in the node no longer exists,
                     // which means the job(s) have already been closed.
-                    // returning false, here because the current close request hasn't actually closed anything.
-                    return new Response(false);
+                    return new Response(true);
                 }
             }
 

plugin/src/main/java/org/elasticsearch/xpack/ml/action/StopDatafeedAction.java

@@ -400,9 +400,9 @@ public class StopDatafeedAction
                     throw org.elasticsearch.ExceptionsHelper
                             .convertToElastic(failedNodeExceptions.get(0));
                 } else {
-                    throw new IllegalStateException(
-                            "Expected [" + request.getResolvedDatafeedIds().length
-                                    + "] number of tasks but " + "got [" + tasks.size() + "]");
+                    // This can happen we the actual task in the node no longer exists,
+                    // which means the datafeed(s) have already been closed.
+                    return new Response(true);
                 }
             }