[webrtc-pc] SLD({type: "offer"}) wo/createOffer should be InvalidModificationError (#2143)

jan-ivar has just created a new issue for https://github.com/w3c/webrtc-pc:

== SLD({type: "offer"}) wo/createOffer should be InvalidModificationError ==
What error should this produce?
```js
await new RTCPeerConnection().setLocalDescription({type: "offer"}); // ?
```
The default `""` sdp matches **[[LastOffer]]**, which is also `""` here since *createOffer* was never called, so we get past the *InvalidModificationError* check, which seems unintentional.

*RTCError*? *InvalidAccessError*? *OperationError*?

I vote we improve the *InvalidModificationError* check to catch it.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2143 using your GitHub account