How to show a user-made selection in a summary slide

Hi all, I am creating an exercise in Storyline in which trainees need to indicate in a long list of our products which product or products they would advice our customers to use. There are about 16 products to choose from and they can choose one to (in theory) all products. They can also change their mind and unselect / select indefinitely until they go to the next page or even go back and change their selection. FYI, in the full exercise, there are more selection slides after this one, but there they only need to select one option, and then it's easy to display their choice.

Before they submit their selection for feedback, I want to show them a summary slide that recaps what they chose in a neat and tidy list without any gaps. The easiest way would be to add a layer to the summary slide that copies the selection slide and hides the non-selected ones, but that would leave me with gaps. I would rather have a textual list that shows their selection. To be more precise, the list would be a (comma separated) text that says "You selected [Product1], [Product 4], [Product 7], etc". I thought adding text variables per product that I then reference to in the summary, but I don't know how to make Storyline disregard empty variables or how to avoid getting stuck in a chaos of nested ifs that nobody can ever make sense of again :-).

FYI, I am getting back into Storyline after a break of more than three years, so I might be missing a simple trick, here. If anyone has an easy fix (or a bit less easy is fine as well :-)), I'd be very grateful.

I can't add my project, unfortunately, as it contains confidential information.

Many thanks in advance.

BR,

Jan.