skipemptyfilesat and parallel didn't play nicely together.

PR: 4347


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@269836 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -237,7 +237,7 @@ public class ExecuteOn extends ExecTask {
                 }
             }
 
-            if (parallel) {
+            if (parallel && (fileNames.size() > 0 || !skipEmpty)) {
                 String[] s = new String[fileNames.size()];
                 fileNames.copyInto(s);
                 File[] b = new File[baseDirs.size()];