fixed NullReferenceException on Android NotificationToXam method

[andrepereiranmbrs]

Description

One Line Summary

Fix for a NullReferenceException on Android when opening a notification. The issue is in the NativeConversion.NotificationToXam(OneSignalAndroid.OSNotification notification) method.

Motivation

Fixes a crash when getting a notification after getting several crash reports on a production app.

NativeConversion.NotificationToXam (Com.OneSignal.Android.OSNotification notification)
System.NullReferenceException: Object reference not set to an instance of an object
NativeConversion.NotificationToXam (Com.OneSignal.Android.OSNotification notification)
NativeConversion.NotificationOpenedResultToXam (Com.OneSignal.Android.OSNotificationOpenedResult result)
OneSignalImplementation+OSNotificationOpenedHandler.NotificationOpened (Com.OneSignal.Android.OSNotificationOpenedResult notificationOpenedResult)
OneSignal+IOSNotificationOpenedHandlerInvoker.n_NotificationOpened_Lcom_onesignal_OSNotificationOpenedResult_ (System.IntPtr jnienv, System.IntPtr native__this, System.IntPtr native_p0)
JNINativeWrapper.Wrap_JniMarshal_PPL_V (_JniMarshal_PPL_V callback, System.IntPtr jnienv, System.IntPtr klazz, System.IntPtr p0)

Affected code checklist

• [ ] Notifications
  • [ ] Display
  • [x] Open
  • [ ] Push Processing
  • [ ] Confirm Deliveries
• [ ] Outcomes
• [ ] Sessions
• [ ] In-App Messaging
• [ ] REST API requests
• [ ] Public API changes

Checklist

Overview

• [x] I have filled out all REQUIRED sections above
• [x] PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• [ ] Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• [ ] I have included test coverage for these changes, or explained why they are not needed
• [ ] All automated tests pass, or I explained why that is not possible
• [ ] I have personally tested this on my device, or explained why that is not possible

Final pass

• [x] Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• [x] I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

---

This change is