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

spec/unit/matrixrtc/CallMembership.spec.ts

@@ -29,9 +29,7 @@ const membershipTemplate: CallMembershipData = {
 function makeMockEvent(originTs = 0): MatrixEvent {
     return {
         getTs: jest.fn().mockReturnValue(originTs),
-        sender: {
-            userId: "@alice:example.org",
-        },
+        getSender: jest.fn().mockReturnValue("@alice:example.org"),
     } as unknown as MatrixEvent;
 }