I'm sure that every league that starts back up will have a plan for dealing with a positive case. But as Mike Tyson famously said, "Everyone has a plan until they get punched in the mouth".
My son raised an interesting question. If the NHL is able to have their 24 team tournament in the summer, how will that affect the next season which will be close to the normal start? I have no clue, but my suspicion is they'll try to make sure that whatever form the next season takes it will include fans in the stands.