type(button): fix icon PropType

app/components/button.js

@@ -70,7 +70,7 @@ type Props = {
   to?: ?string,
   variant?: 'primary' | 'secondary',
   disabled?: boolean,
-  icon?: string,
+  icon?: string | null,
   className?: string,
   isLoading?: boolean,
 };
@@ -88,24 +88,21 @@ export const Button = ({
   if (to && onClick) throw new Error('Should define either "to" or "onClick"');
 
   const props = {
-    onClick, disabled: disabled || isLoading, icon: null, className,
+    onClick,
+    disabled: disabled || isLoading,
+    icon: null,
+    className,
   };
   const buttonLabel = isLoading ? 'Loading...' : label;
 
   const component = variant === 'primary' ? (
     <Primary {...props}>
-      {icon
-        ? <Icon src={icon} />
-        : null
-      }
+      {icon ? <Icon src={icon} /> : null}
       {buttonLabel}
     </Primary>
   ) : (
     <Secondary {...props}>
-      {icon
-        ? <Icon src={icon} />
-        : null
-      }
+      {icon ? <Icon src={icon} /> : null}
       {buttonLabel}
     </Secondary>
   );