Version 0 `m.call.hangup` to reject an `m.call.invite` is ignored

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**Describe the bug**
When sending a version 0 `m.call.hangup` event to reject a call after receiving an `m.call.invite` from Element Web, the call is not ended on Element Web's side.

These messages show up in the logs:

```
Hangup received for call ID abcdef123
Ignoring message from party ID undefined: our partner is undefined
```

**To Reproduce**
Steps to reproduce the behavior:
1. Open a 1-to-1 room in Element Web
2. Click on 'Audio Call'
3. From another client send an `m.call.hangup` event with the proper `call_id` and `version: 0`.
4. The call is still "ringing" on Element Web.

**Expected behavior**
The call should end on Element Web's side.

**Desktop (please complete the following information):**
- OS: Linux Fedora 36
- Browser: Firefox
- Version: 100.0
- Element Web v1.10.12

**Additional context**
Looking at the code, the error seems to be that:
1. `this.opponentPartyId` is `undefined` because no opponent was chosen yet.
2. [When the version is 0, `msgPartyId` is set to `null` in `partyIdMatches()`](https://github.com/matrix-org/matrix-js-sdk/blob/25afb7cb3cc612656458c00569cf03c42277aae2/src/webrtc/call.ts#L2196),
3. [`partyIdMatches()` returns `false`](https://github.com/matrix-org/matrix-js-sdk/blob/25afb7cb3cc612656458c00569cf03c42277aae2/src/webrtc/call.ts#L2197) because `null !== undefined`,
4. The condition fails in [`onHangupReceived()`](https://github.com/matrix-org/matrix-js-sdk/blob/25afb7cb3cc612656458c00569cf03c42277aae2/src/webrtc/call.ts#L1856) so the event is ignored.

Start in src/webrtc/call.ts at partyIdMatches() and onHangupReceived(), as identified in the issue. Trace the version 0 hangup path when opponentPartyId is undefined, then reproduce the one-to-one call scenario or add focused coverage if the existing test structure is clear. Done means a version 0 m.call.hangup with the call_id ends the ringing call in Element Web.

Written by the indexing model from the issue text.