title: If dataTaskPublisher has non-nil data, what is the error type then?
body: <p>Before with </p>
  
  <p><code>URLSession.shared.dataTask(for request: URLRequest) { data, error, response in ... }</code></p>
  
  <p>The <code>data</code> field as well as the others could be nil. Now with Combine's </p>
  
  <pre><code>URLSession.shared.dataTaskPublisher(for: urlRequest)
  </code></pre>
  
  <p>The (data, response) tuple is no longer of optional types. I realize that means that we must have it as a URLError that we can catch in the <code>sink(receiveCompletion: { ... }</code>, but how can I differentiate between no data and any other error?</p>