fix(runtime): deliver a posted undefined as undefined, not null

[adrian-niculescu]

postMessage(undefined) arrives as null everywhere a message is delivered: a MessagePort, a BroadcastChannel, a Worker and its global scope, and a node:worker_threads parentPort. Node and browsers deliver undefined.

Every delivery path builds its event with new MessageEvent(type, { data, ports }), and the constructor's init dictionary turns an undefined data into null, as Web IDL requires. Delivery now goes through an internal createMessageEvent that stores the payload as given, the same fix as NativeScript/ios#477. The native messageerror paths pass null instead of undefined, so that event keeps its default. A worker's messageerror now carries the deserialization failure as its data, as a port's already does and as NativeScript/ios#489 does on iOS.

Stacked on #2043. The new specs in testMessaging.js fail on that branch and pass with this change, and the full device suite passes.

[coderabbitai]

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