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/structures/UserMenu.tsx

@@ -429,7 +429,7 @@ export default class UserMenu extends React.Component<IProps, IState> {
         );
     };
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const avatarSize = 32; // should match border-radius of the avatar
 
         const userId = MatrixClientPeg.get().getUserId();