title: HKSourceQuery not returning values when using with predicateForWorkouts
body: <p>I am trying to load all sources for cycling workouts. For this, I am using a <code>HKSourceQuery</code> like in the following code block:</p>
  <pre><code>let workoutPredicate = HKQuery.predicateForWorkouts(with: .cycling)
  let query = HKSourceQuery(sampleType: .workoutType(), samplePredicate: workoutPredicate) { (query, samples, error) in
      // print(samples, error) prints nil, nil
  }
  </code></pre>
  <p>However, unexpectedly, bot <code>error</code> and <code>samples</code> are <code>nil</code>. There exist cycling workouts in the health store, both from my own app and from an Apple Watch. Passing <code>samplePredicate: nil</code> returns sources. Passing <code>samplePredicate: sourcePredicate</code> with <code>let sourcePredicate = HKQuery.predicateForObjects(from: .default())</code> works. Only querying for all cycling workouts does not work.</p>
  <p><strong>How do I load all sources for cycling workouts from HealthKit?</strong></p>