Accessibility: make screen readers not to read label's service parts in Form (T1320128) (#32321)

Author: pharret31

packages/devextreme-scss/scss/widgets/base/_form.scss

@@ -155,7 +155,7 @@
 .dx-field-item-optional {
   .dx-label > span {
     &::after {
-      content: attr(data-mark);
+      content: attr(data-mark) / '';
     }
   }
 }