Make read receipts handle nullable roomMembers correctly (#8410)

* make readReceipt roomMember nullable, as it should be
* add fallback click interaction for read receipts of unknown users

src/components/views/avatars/MemberAvatar.tsx

@@ -30,7 +30,7 @@ import SettingsStore from "../../../settings/SettingsStore";
 import { MatrixClientPeg } from "../../../MatrixClientPeg";
 
 interface IProps extends Omit<React.ComponentProps<typeof BaseAvatar>, "name" | "idName" | "url"> {
-    member: RoomMember;
+    member: RoomMember | null;
     fallbackUserId?: string;
     width: number;
     height: number;