Price set option limit = 0 should mean no spaces, rather than no limit
If you set a price set option limit to 0, this is the same as not specifying a limit. I would expect that a limit of 0 would mean there are no spaces. Setting the limit to 0 is different than disabling the price set option, as disabling makes it so the option does not appear on public forms, while I expect setting the limit to 0 would just make it sold out, but still appear. I can't think of a good reason you would want 0 to be the same as no limit, but maybe others can?
Will submit PR unless there are objections. Otherwise, will add help text.