Can wmcontrol support "pu_dataset = None"?

Hi, 

I'm submitting the special request where NoPileup is considered for the DIGI step (ReDigi type). I found that "pu_dataset = None" is not supported with current wmcontrol, so I just delete this line out from conf file before submit.

I think it should be great if wmcontrol can support "pu_dataset = None". To me, it's more clear to say "None", than nothing, or comment line.
