Fix disappearing Youtube video

- on this page https://www.gov.uk/government/publications/electronic-travel-authorisation-eta-help-videos/what-is-an-eta-video there is only an embedded video, which does not appear on mobile
- this seems to be due to the `.offset-empty-contents-list` class, which floats the main column right if there is no contents list on the left to push it across
- however below desktop this styling persists, when all the grid columns should have collapsed
- somehow if there is only a Youtube video present in this floated element, it produces an element with zero width, so the video is effectively invisible
- adjusting the class to only apply on desktop seems to fix the issue

app/assets/stylesheets/views/_html-publication.scss

@@ -37,11 +37,15 @@
   }
 
   .offset-empty-contents-list {
-    float: right;
+    // stylelint-disable max-nesting-depth
+    @include govuk-media-query($from: desktop) {
+      float: right;
 
-    .direction-rtl & {
-      float: left;
+      .direction-rtl & {
+        float: left;
+      }
     }
+    // stylelint-enable max-nesting-depth
   }
 
   .meta-data {