Check null value in form field

Hi,

Just started with this awesome module!

I am trying to create a rule where 'form element fieldname value check' needs to look at the field, and do something only if it is a null value. Entering 'null' in the value text area does not do the job. Leaving it blank is not possible as it is a mandatory field. How do I do this with rules, without hacking the other module that is creating the form?

Thanks
Srini

P.S.

I tried putting the condition (with negate)

in the value text area as suggested in http://drupal.org/node/675222 and it worked, but it is redirecting every time, even when the field value is not null.

Thanks