stop generating these "part-XXXX" empty files when using MultipleOutputs in mapreduce job

[S. Zhou]

I use MultipleOutputs so the output data are no longer stored in files 
"part-XXX". But they are still generated (though empty). Is it possible to stop 
generating these files when running MR job? (BTW, my MR job only has mapper). 
Thanks

Senqiang