Trigger workflow when field is left empty

Hi

I'd like to trigger a workflow when a field has been left empty, e.g. my application has a reference to another app. This reference field is not mandatory. However I'd like to create a task reminding the user to fill in the reference at a later point in time if the field is left empty when the item is created.

Are there any means to do that with workflows in Podio?

Regards

Volker