v0.2002 breaks methods that correctly return "" as a false value

As an unintended consequence of the fixes merged in #135, some subs can no longer return false, even though they should be able to.

 `$elem->get_attribute("data-missing-attr")`, for example, would return `""` in v0.2001, but now the `""` gets caught up and transformed into `"Server returned status code '.$response->code.' but no data'"`instead.

Perhaps we need to distinguish between endpoints where the return value could be interpreted as a boolean, and endpoints where a return value of `""` is the expected value.