Getter operations return null without proper IDL

[saschanaz]

For example, the SpeechRecognitionResult's getter operation says:

If index is greater than or equal to length, this returns null

But the IDL does not indicate so:

[Exposed=Window]
interface SpeechRecognitionResult {
    readonly attribute unsigned long length;
    getter SpeechRecognitionAlternative item(unsigned long index);
    readonly attribute boolean isFinal;
};

The return types must be nullable here. I'm not sure how Blink allows returning null here, though?