I did a quick test....if your array is empty prior to the ListBuilder, then you are correct, the form will not let you exit without entering at least one item into the list, even if you chose Optional for your output path.
What I did notice, though, is if your array has at least one item in it before the ListBuilder, then the form
will let you exit without adding any additional items.
So it seems that the real restriction is on having at least one item in the array, even if you specified that the component is optional. I'm not sure if that's a bug or just a design quirk, but that seems to be the way it's set up. I don't know how widely used this component is, but it is a nice way to build a simple text array on the fly from a web form.