Fix Series.hasValues, add test coverage

> Are classifications actually asserted in automated unit tests? I'd be hesitant to rely on this telling us we have good coverage if it won't break on us if cases somehow stop being hit over time

Classifications seem mostly to be useful when initially developing properties. With a few initial runs, you get some empirical evidence on the rough distribution of values generated. And after that, you can let randomness take its course—another reason why I think it’s good to let generators ‘roam’ as freely as possible.