[native] Don't dispatch an undefined token

Summary:
CommCoreModule can return an undefined token. We're replacing it with null, because otherwise redux-persis won't persist it.

https://linear.app/comm/issue/ENG-6849/expected-keys-not-rehydrated-[commservicesaccesstoken]

Test Plan: Without this diff: restart the app - the warning should be shown. Apply this diff and restart the app - the warning shouldn't appear.

Reviewers: inka, atul, varun

Reviewed By: inka, varun

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11128