0 isn't a problem, 1 is. It isn't in the main workflow, but in a call to a Render. I added an input variable of type Map to the Render. In the Advanced tab of my Document Form, I pass the main workflow variable of type Map to the Render. I suppose I could create 2 variables in the Render, a Map and a single document. In the script step that moves the data from the List to the Map, if there is only 1, put it in the document variable instead. Then I would have to deal with it in the Render.
I just ran it again with 1 attachment and it worked. I haven't changed anything in the Render or Advanced tab of the variable, just adding in the description, I guess I won't complain unless it starts breaking again later.