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/RoomList.tsx

@@ -687,7 +687,7 @@ export default class RoomList extends React.PureComponent<IProps, IState> {
         [...treeItems].find((e) => e.offsetParent !== null)?.focus();
     }
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const sublists = this.renderSublists();
         return (
             <RovingTabIndexProvider handleHomeEnd handleUpDown onKeyDown={this.props.onKeyDown}>