Fix empty classname resulting in undefined class of result element

src/List.tsx

@@ -17,10 +17,10 @@ export default function List({
 
    if (divider) cl += "ht-list-divider ";
 
-   cl += className;
+   cl += className || "";
 
    return (
-      <ul className={className} {...props}>
+      <ul className={cl} {...props}>
          {children}
       </ul>
    );