Fix Series.hasValues, add test coverage

Yeah… too many different generators and you can’t keep track of what they actually generate. If you put the fine-tuned restrictions in the property themselves, then at least you can see them right there, rather than needing to look up the generator definition.

And if you have more than one case in a test, you can always use the classification to ensure that each case is actually being hit.

That doesn’t solve the problem of “which case broke”… so I guess more that two is probably bad.