How can I make a question mandatory if a previous question is unanswered?

Hello everyone,
For my experiment, I have a trial where I want participants to either put items in the box, or check the option that says "this box should be left blank." The box with items and the "this box should be left blank" option belong to separate questions (a "Pick, group and rank" question and a multiple choice question).
How can I turn on a logic where checking the "this box should be left blank" option is mandatory if a participant does not put any items in the box (i.e., if the previous question is unanswered)? I would appreciate any suggestions and let me know if you have any clarification questions.