IssueTracker

Artifact used: android.arch.work:work-runtime-ktx
Version used: 1.0.0-alpha02
Theme used: N/A
Devices/Android versions reproduced on: N/A

- Relevant code to trigger the issue. N/A
- A screenrecord or screenshots showing the issue (if UI related). N/A


WorkManager has nullability annotations for input parameters, but not for return types. Consumers, especially those using Kotlin, are more likely to make bad assumptions about whether a return value is null or not, leading to potential crashes.

Example:

```
    /**
     * Gets a {@link LiveData} of the {@link WorkStatus} for a given work id.
     *
     * @param id The id of the work
     * @return A {@link LiveData} of the {@link WorkStatus} associated with {@code id}
     */
    public abstract LiveData<WorkStatus> getStatusById(@NonNull UUID id);
```

If the given id is not found, what is returned? A LiveData that immediately emits a Failure WorkStatus? A null LiveData? Or is an exception thrown?