Clarify target object of Client.postMessage can be null

This change addresses the issue described in the second paragraph of
#1042 (comment).
This change gets |destination| variable initialized to null if the
ServiceWorkerContainer object does not exist.

Fixes #1042.

docs/index.bs

@@ -1028,7 +1028,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       The <dfn method for="Client"><code>postMessage(|message|, |transfer|)</code></dfn> method *must* run these steps:
 
         1. Let |sourceSettings| be the <a>context object</a>'s <a>relevant settings object</a>.
-        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=].
+        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=], if any; otherwise let it be null.
         1. If |destination| is null, <a>throw</a> an "{{InvalidStateError}}" {{DOMException}}.
         1. Let |serializeWithTransferResult| be <a abstract-op>StructuredSerializeWithTransfer</a>(|message|, |transfer|). Rethrow any exceptions.
         1. Add a <a>task</a> that runs the following steps to |destination|'s [=ServiceWorkerContainer/client message queue=]:

docs/v1/index.bs

@@ -966,7 +966,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       The <dfn method for="Client"><code>postMessage(|message|, |transfer|)</code></dfn> method *must* run these steps:
 
         1. Let |sourceSettings| be the <a>context object</a>'s <a>relevant settings object</a>.
-        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=].
+        1. Let |destination| be the {{ServiceWorkerContainer}} object whose [=ServiceWorkerContainer/service worker client=] is the <a>context object</a>'s [=Client/service worker client=], if any; otherwise let it be null.
         1. If |destination| is null, <a>throw</a> an "{{InvalidStateError}}" {{DOMException}}.
         1. Let |serializeWithTransferResult| be <a abstract-op>StructuredSerializeWithTransfer</a>(|message|, |transfer|). Rethrow any exceptions.
         1. Add a <a>task</a> that runs the following steps to |destination|'s [=ServiceWorkerContainer/client message queue=]: