[native] Consider null notification tag in CommNotificationsHandler

Summary:
I noticed an error while testing my React Native 0.70 branch on Android: https://web.comm.app/upload/3959996/63ded52241732e2f

My guess is that it happened because the "empty notification" generated by react-native-background-upload (context here) does not have a tag.

Either way, the API is clear that this value can be null, so we should check for it.

Test Plan: I tested generating a rescind notif on a client after that client uploaded a message in the background and the issue did not reoccur

Reviewers: marcin, tomek

Reviewed By: tomek

Subscribers: atul

Differential Revision: https://phab.comm.dev/D5889