[Notifications]

### Device / Emulator and OS
Please complete the following information for each phone and/or emulator you're experiencing this bug on:
- Device: Samsung
- OS: Android 11

### Describe the bug

There is a null pointer exception in the `dk.cachet.notifications.NotificationListener`:

```
E/AndroidRuntime(23308): FATAL EXCEPTION: main
E/AndroidRuntime(23308): Process: dk.cachet.notifications_example, PID: 23308
E/AndroidRuntime(23308): java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.String java.lang.CharSequence.toString()' on a null object reference
E/AndroidRuntime(23308): at dk.cachet.notifications.NotificationListener.onNotificationPosted(NotificationListener.java:40)
E/AndroidRuntime(23308): at android.service.notification.NotificationListenerService.onNotificationPosted(NotificationListenerService.java:375)
E/AndroidRuntime(23308): at android.service.notification.NotificationListenerService$MyHandler.handleMessage(NotificationListenerService.java:2166)
E/AndroidRuntime(23308): at android.os.Handler.dispatchMessage(Handler.java:106)
E/AndroidRuntime(23308): at android.os.Looper.loop(Looper.java:246)
E/AndroidRuntime(23308): at android.app.ActivityThread.main(ActivityThread.java:8512)
E/AndroidRuntime(23308): at java.lang.reflect.Method.invoke(Native Method)
E/AndroidRuntime(23308): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
E/AndroidRuntime(23308): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
```

### To Reproduce

Running the example app. Turning the screen off/on.