I would wager that it is far far more important that you intimately understand a market or have at least demonstrated the capacity to do so, than to have an idea.
In the end, users is just another word for audience. Before you can acquire users, you need to build with an audience in mind. It may not always be your intended audience that adopts your product, but being audience-centric prepares you for catering effectively to whoever does end up using your product.
Were I designing the application for no-idea applicants, I would put a bunch of questions related to a certain audience and the pains they experience and ways in which that audience can be served better. An idea is just a hypothesis (a question with direction). Understanding the audience is observation. It's more important to be good at observing because it leads you to know when your hypothesis is wrong and to new hypotheses
