Don't use event.sender in CallMembership (#3793)

* Don't use event.sender in CallMembership

I fell into another js-sdk trap: this is "only guaranteed to be set
for events that appear in a timeline" and not state events. It does
not say why. We only ever used it to get the sender user ID anyway,
so just use getSender().

* Fix test

src/matrixrtc/MatrixRTCSession.ts

@@ -120,7 +120,7 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
         callMemberships.sort((a, b) => a.createdTs() - b.createdTs());
         logger.debug(
             "Call memberships, in order: ",
-            callMemberships.map((m) => [m.createdTs(), m.member.userId]),
+            callMemberships.map((m) => [m.createdTs(), m.sender]),
         );
 
         return callMemberships;