Shorten if notations so that property order can be applied

scss/_dropdown.scss

@@ -166,9 +166,7 @@
     pointer-events: none;
     background-color: transparent;
     // Remove CSS gradients if they're enabled
-    @if $enable-gradients {
-      background-image: none;
-    }
+    background-image: if($enable-gradients, none, null);
   }
 }