workflow - check for default value

I have a workflow for the issues to be color coded based on the value of Due Date.  Because Due Date is not a required field in my organization, there are a bunch of issues where the Due Date is set to the default value of "N/A".

How would I check against that in a workflow?  I'd like to define a color for that state as well.

Currently, the code in the workflow(I got it off this site, I think, and it works) looks like;


rule ColorOnDeadLineApproach

when <issue created or updated> {
if (!issue.State.isResolved) {
  if (!issue.State.isResolved) {
   issue.DeadlineApproach = {More than a week};
  }

  if (now + 4 days > Due Date) {
   issue.DeadlineApproach = {A week};
  }
  ...
}
}

Thanks in advance!