Fix invalid CSS selector syntax (#2303)

https://jigsaw.w3.org/css-validator/ complains that "The selector :empty can't appear after the pseudo-element selector ::after". Switching their places, however, will validate.

src/sass/plugins/ads.scss

@@ -36,7 +36,7 @@
     z-index: 3;
   }
 
-  &::after:empty {
+  &:empty::after {
     display: none;
   }
 }