Making text-entry fields mandatory

Right now I have a slide with multiple text-entry fields. Each of them saves what the learner inputs to reference later in the presentation. The format is a bit like mad-libs, where you enter write multiple short responses that are shown to you later in the presentation.

The problem is, I'd really like these fields to be mandatory. If you think back to something like mad-libs again, the final result is not so impressive if you leave some (or all) of the fields blank. Unfortunately, I can only see ways to evaluate if the fields contain specific values. Is there any way to evaluate whether the user has typed anything into the fields? A wildcard, or something?

Also,  when making general quiz slides, I can change the text of the "correct" and "incorrect" alerts, but I don't see how to change the, "You must answer the question before moving on" alert. Where can I change this?