HDFS-13261. Fix incorrect null value check. Contributed by Jianfei Jiang.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/tools/AdminHelper.java

@@ -145,7 +145,7 @@ public class AdminHelper {
     private final Command[] commands;
 
     public HelpCommand(Command[] commands) {
-      Preconditions.checkNotNull(commands != null);
+      Preconditions.checkNotNull(commands, "commands cannot be null.");
       this.commands = commands;
     }