Conform more of the code base to strict null checking (#10147)

* Conform more of the code base to strict null checking

* More strict fixes

* More strict work

* Fix missing optional type

* Iterate

src/components/views/rooms/RoomHeader.tsx

@@ -715,7 +715,7 @@ export default class RoomHeader extends React.Component<IProps, IState> {
         return <div className="mx_RoomHeader_name mx_RoomHeader_name--textonly">{roomName}</div>;
     }
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const isVideoRoom = SettingsStore.getValue("feature_video_rooms") && calcIsVideoRoom(this.props.room);
 
         let roomAvatar: JSX.Element | null = null;