`PullPublisher` logic is branching on the `throwable == null` condition on several places – this increases code size and complexity. Plus, all `PullPublisher::new` call sites know at compile time whether `throwable` is null or not. `PullPublisher` should have distinct factory methods (or constructors) each accepting only either a `CheckedIterable` or a `Throwable`, but not both.

Related discussions:

- https://github.com/openjdk/jdk/pull/26876/files#r2329782120
- https://github.com/openjdk/jdk/pull/26876/files#r2360235119