You can simply add a validation rule to your field in the Nintex Form.To make it even more simple you can just add a set for validation setting in the configure action as shown below.
Stephan mentioned he is looking to check if something is empty, not necessarily for validation purposes. This may work if it is for validation, but otherwise, another path is best.
It's more complex than this. This field isn't required but I need to know if it's empty. If it's filled I create list items for a dependent list and then clear it out and update a status field (read only) which then contains the last two status items by date.
Here is the simplest approach.
I’ve added two fields to a custom list: Details (multi-line text input) and Status (choice field in Display Mode). A workflow will run on each item and update Status to Empty or Not Empty depending on the field value. A condition is used to determine how the list should be updated.
The condition is configured using the “is empty” condition.
Once the workflow runs, the Statuses have been updated accordingly. In your case, you’d use the branches to create your list items and update the required fields.
are you sure the field is really empty? doesn't it contain eg. space(s).
'is empty' check should normally work. I've tried it with both plaintext and reach text, works for both for me.
if it really doesn't work for you, you can check for emptiness with regular expression.