Fix up new input stuff since input thread won't always exist

PR:	16891
Submitted by:	paul at priorartisans


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@274019 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/PumpStreamHandler.java

@@ -131,7 +131,9 @@ public class PumpStreamHandler implements ExecuteStreamHandler {
     public void start() {
         outputThread.start();
         errorThread.start();
-        inputThread.start();
+        if (inputThread != null) {
+            inputThread.start();
+        }
     }
 
     public void stop() {