You can either create a new row in your data set start 10 end 11 with message ‘No Exams till 11:00’
Which will work fine, but I don’t think it’s that good - you’ll probably have a lot of ‘free’ hours and different each day etc.
I’d go to ‘Edit ticker’ -> advanced tab -> at the bottom there is “A message to display when no data is returned from the source” -> so there you could write something like “Room is currently free” or something like that.
This way this message will appear when there is no data returned from the dataset, so each time the room is free that message should be displayed.