minor: do not flood log with redundant messages when nothing is done.

SVN: 20943

datastore_server/source/java/ch/systemsx/cisd/etlserver/postregistration/TaskExecutor.java

@@ -136,7 +136,7 @@ public class TaskExecutor
     {
         Log4jSimpleLogger logger = new Log4jSimpleLogger(operationLog);
         File[] files = cleanupTasksFolder.listFiles(FILTER);
-        if (files != null)
+        if (files != null && files.length > 0)
         {
             operationLog.info("Perform " + files.length + " clean up task.");
             for (File file : files)