Async JavascriptBinding - Return null instead of empty object for async Task methods

[amaitland]

For async Task methods (those without a generic return type) the result passed back to Javascript is effectively {}. Rather than creating an unnecessary object as the method didn't return a result I'm changing the behaviour to return null instead.

Technically this is a breaking change as it changes the behaviour, expecected impact on users is extremely low.

public async Task WaitBeforeReturnAsync(int milliseconds)
{
	await Task.Delay(milliseconds);

	Debug.WriteLine("Delayed in ms:" + milliseconds);
}

Technically the Task returns VoidTaskResult which doesn't have any properties.