[w3c/ServiceWorker] Clarify target object of Client.postMessage can be null (#1274)

This change addresses the issue described in the second paragraph of
https://github.com/w3c/ServiceWorker/issues/1042#issuecomment-270043959.
This change gets |destination| variable initialized to null if the
ServiceWorkerContainer object does not exist.

Fixes #1042.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/ServiceWorker/pull/1274

-- Commit Summary --

  * Clarify target object of Client.postMessage can be null

-- File Changes --

    M docs/index.bs (2)
    M docs/v1/index.bs (2)

-- Patch Links --

https://github.com/w3c/ServiceWorker/pull/1274.patch
https://github.com/w3c/ServiceWorker/pull/1274.diff

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