[heycam/webidl] Allow [TreatNullAs=EmptyString] for USVString? (#340)

This is used in https://w3c.github.io/webrtc-pc/#peer-to-peer-data-api

```WebIDL
partial interface RTCPeerConnection {
    readonly attribute RTCSctpTransport? sctp;
    RTCDataChannel createDataChannel([TreatNullAs=EmptyString] USVString label,
                                     optional RTCDataChannelInit dataChannelDict);
    attribute EventHandler ondatachannel;
};
```

However, it's not allowed per Web IDL:
https://heycam.github.io/webidl/#TreatNullAs

> The `[TreatNullAs]` extended attribute must not be associated with a type that is not DOMString.

Is there a reason for this restriction?

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