[ML] Only log long time unassigned message if not empty (elastic#100417)

Followup to elastic#100154

The log message about unassigned jobs is simply spam if
there are no unassigned jobs, so it should only be logged
if there's at least one item in the list.

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

@@ -267,8 +267,9 @@ private void logLongTimeUnassigned(Instant now, ClusterState state) {
         }
 
         List<String> itemsToReport = findLongTimeUnassignedTasks(now, tasks);
-
-        logger.warn("ML persistent tasks unassigned for a long time [{}]", String.join("|", itemsToReport));
+        if (itemsToReport.isEmpty() == false) {
+            logger.warn("ML persistent tasks unassigned for a long time [{}]", String.join("|", itemsToReport));
+        }
     }
 
     /**