Fix compatability with v0 calls
Fix compatability with v0 calls

https://github.com/matrix-org/matrix-js-sdk/pull/1567 introduced a
bug where we'd leave opponentPartyId undefined, but we compared it
to null later when testing for its presence.

Fixes https://github.com/vector-im/element-web/issues/16239

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk