Fix empty classname resulting in undefined class of result element

src/IconButton.tsx

@@ -5,7 +5,7 @@ export default function IconButton({
    ...props
 }: JSX.HTMLAttributes<HTMLButtonElement>) {
    return (
-      <button className={"ht-btn ht-btn-icon " + className} {...props}>
+      <button className={"ht-btn ht-btn-icon " + (className || "")} {...props}>
          {children}
       </button>
    );