test_zero: fix comment re 0-weight handling

        // in the presence of items whose weight is 0.

        //

        // We think that the behavior is:

        //   - An item with weight 0 is never returned.

        //   - If all items have weight 0, choose_weighted returns an error.

        //   - If all items have weight 0, sample_weighted returns an empty list.

        //   - If we request n items from sample_weighted,