How to see no value selected in custom field

Does anyone know how to see if a custom field has not been set in the example below?From: Sean stroke_of_death@yahoo.com
To: Brian Gallew geek+rt@cmu.edu; RT-Users@lists.bestpractical.com
Sent: Thursday, September 20, 2007 5:41:46 PM
Subject: Re: [rt-users] Custom reports

All,

I implemented the attached report generator, and it works great with one exception.
I would like to be able to see which tickets do not have a value in that custom field. Meaning, the page shows a count of how many tickets have value x, and value y in that custom field, I would like an additional count of how many have not set something in that custom field and after a few hours, I still cant figure out how to do it so I thought I would ask. I have reattched the file for convienience.

Thanks,
Jeff

From: Brian Gallew geek+rt@cmu.edu
To: RT-Users@lists.bestpractical.com
Sent: Wednesday, September 12, 2007 6:42:48 AM
Subject: Re: [rt-users] Custom reports

Fernando Fugita wrote:

Hello,

I need to create a custom report, showing the number of tickets for
certain type (a custom field of the RT), in a determined period.
I tried the RTx-Statistics, but was not useful.

You really have to write it yourself. Attached is a report that we use
at Carnegie Mellon - Qatar.

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