[native] Fix rescinds and badge-only notifs on iOS

Summary:
D9178 broke this by requiring [content.userInfo[@"id"] to be non-nil in NotificationService.mm.

This diff makes it possible for that value to be nil without crashing the NSE.

Test Plan:

1. I managed to reproduce the issue in my local environment
2. I then attached the Xcode debugger to the NSE, following the instructions here
3. After my diff, I confirmed that the issue no longer occurs
4. I also confirmed in the debugger that the value of [content.userInfo[@"id"] was in fact nil

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D9248