[ServiceWorker] Make FetchEvent.prototype.client nullable (#631)

E.g. for `sendBeacon()` or `ping=""` there might no longer be a client environment to expose.

(Incidentally, those also do not really care about a response, do we want to expose that differently?)

---
Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/631