[web] Don't crash if electron.fetchDeviceToken not defined in useCreateDesktopPushSubscription

Summary: Context in ENG-6000. This diff fixes a regression introduced in D9970.

Test Plan: I deployed this fix to my keyserver in production and it resolved the issue described on Linear.

Reviewers: marcin, michal

Reviewed By: marcin

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10129