Handle nulled duplicate jobs

I'm confused by these changes... For example, prepareJob() currently cannot return NULL? In which case can it return a NULL job now?

Yes, Queue::prepareJob() should be able to return NULL, meaning 'discard this job'. This makes sense because prepaeJob() relies on job type plugin's handleDuplicateJobs() method, which has the design feature that it can return nulls, meaning "discard the current because it's the duplicate of an already queued one".

Queue:

  protected function prepareJob(Job $job): ?Job {
    ...
      if ($duplicates = $this->getBackend()->getDuplicateJobs($job)) {
        $job = $job_type_plugin->handleDuplicateJobs($job, $duplicates, $this->getBackend());
      }
    }
    return $job;
  }

JobTypeInterface:

  /**
   * Handles existing jobs detected as duplicates when enqueing a new job.
   *
   * A function can be used to execute a range of different strategies with
   * regard to duplicate jobs:
...
   * - to discard the new job and leave the duplicate intact, return NULL.
...
   * @return \Drupal\advancedqueue\JobResult|null
   *   A new job to enqueue on the backend, or null if no new job should
   *   be enqueued.
   */
  public function handleDuplicateJobs(Job $job, array $duplicates, BackendInterface $backend): ?Job;