You results file should not be empty, even if the trial fail to record the relevant bits: you should at least have a report that a run took place. Did you make sure to take your experiment until the end (ie. until you get a confirmation message: “The results were successfully sent to the server. Thanks!”)?
The code above already includes a trial without a comprehension question, namely trial 2. As you can tell from the table, you just leave the Question column empty and it will skip that bit. So just don’t write anything into Question in the rows of your filler table where you don’t want a comprehension question to appear