Specify null value

Hi,

I'm using the last version of this script:

http://gallery.technet.microsoft.com/scriptcenter/Reset-password-for-all-412fbc72

to modify the password for school pupils.  The youngest children in the school need no password and not including anything in the inverted commas doesn't seem to work.  Can anyone tell me what I need to do to get the script to recognise a null?

Regards,

Paul.