Check against 'workerRegistration.pushManager' before creating push subscription on web

Summary:
This differential solves https://linear.app/comm/issue/ENG-6006/. Even though web documentation doesn't allow pushManager property of workerRegistration
to be nullish, there are some recources that suggest it could actually be the case so we add additional check to protect against the issue.

Test Plan:
Check that web notifications are working properly. We can't reproduce the issue as it is probably some race condition so we can't directly test this change. However
this diff should protect against it since we won't let access pushManager properties in case it is nullish.

Reviewers: michal, varun, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10351