Fix form editor proptypes warning on submit button

[MAILPOET-3527]

assets/js/src/form_editor/blocks/submit/edit.tsx

@@ -17,7 +17,7 @@ type Props = {
   attributes: {
     label: string;
     styles: InputBlockStyles;
-    className: string;
+    className: string | null;
   };
   setAttributes: (attribute) => void;
 };