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/emojipicker/Category.tsx

@@ -69,7 +69,7 @@ class Category extends React.PureComponent<IProps> {
         );
     };
 
-    public render(): JSX.Element {
+    public render(): React.ReactNode {
         const { emojis, name, heightBefore, viewportHeight, scrollTop } = this.props;
         if (!emojis || emojis.length === 0) {
             return null;