Don't add nullish id field to badgeonly notification payload

Summary:
The id field of Android notification must be either string or be absent at all. Currently if notification doesn't have id field
encryptAndroidNotification function puts nullish value to its encrypted form. This was the casue of
https://linear.app/comm/issue/ENG-6051/encrypted-badge-updates-on-android-are-broken.

Test Plan:

1. Add console.log to fcmSInglePush method to log if sending notifications fails.
2. Build Android app and send badge only notification.
3. Ensure no error is logged

Reviewers: tomek, michal

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10326