XWIKI-13987: Navbar height is increased by 5px on small device (<768px)

* Removed the space used by the left navbar when it has no children (xwiki standard's default)

xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/less/action-menus.less

@@ -45,6 +45,10 @@
       float: left;
       margin-left: @navbar-padding-horizontal;
       margin-right: @navbar-padding-horizontal;
+      /* Without this rule, the left navbar will cause unexpected layout shifts even when it's empty. */
+      &:not(:has(> *)) {
+        margin-top: 0;
+      }  
     }
   }
 }