Creating task/macro wrappers

Hello Ant users,

I'm trying to create a <macrodef> wrapper over <testng>.  The idea is
to have my macro expose the same attributes that <testng> supports,
with as close semantics as possible.

Unfortunately, I have no idea how to reproduce the presence/absence
behavior.  For instance, <testng> accepts a "timeout" integer
attribute.  So in my macro I would do:

<attribute name="timeout" default=""/>

And then calling TestNG would be:

<testng timeout="@{timeout}">...</testng>

But this turns out not to be the same behavior as NOT specifying the
timeout attribute.  In this case, Ant fails because empty string does
not parse to an integer.

Is there a way to achieve my goal?  I would like to avoid writing Java
code.  But I don't mind resorting to Javascript in the build file
(<script> or <scriptdef>).

Thanks in advance.

-- 
Isaac Jurado

"The noblest pleasure is the joy of understanding"
Leonardo da Vinci