How to ask user to affirm that a blank value is intended?

I have a form with several optional text fields, and a workflow in which a user might reasonably save a partially completed form and return to complete it later.

I want to prevent errors where a user leaves an optional field empty by accident. I want the user to say "I really mean to leave this field empty" before the form is submitted.

For example, one can enter a list of departments (comma-delimited list in a free text field) to enable filtering. I would like a user who sets up a job with no department filter to take an affirmative action, like selecting "No department filter," rather than simply leaving the department text box empty.

Are there established patterns for this kind of thing?