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/elements/EventTilePreview.tsx

@@ -111,7 +111,7 @@ export default class EventTilePreview extends React.Component<IProps, IState> {
         return event;
     }
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const className = classnames(this.props.className, {
             mx_IRCLayout: this.props.layout == Layout.IRC,
             mx_EventTilePreview_loader: !this.props.userId,