Fix empty classname resulting in undefined class of result element

src/Button.tsx

@@ -5,7 +5,7 @@ export enum ButtonFormats {
    PRIMARY,
    SECONDARY,
    ERROR,
-   SUCCESS
+   SUCCESS,
 }
 
 export interface IButtonProps extends JSX.HTMLAttributes<HTMLButtonElement> {
@@ -20,7 +20,7 @@ export default function Button({
 }: IButtonProps) {
    let cl = "ht-btn";
 
-   if (className) cl += " " + className;
+   if (className) cl += " " + (className || "");
 
    switch (format) {
       case ButtonFormats.DEFAULT: