Fix margins around service navigation mobile toggle

Fixes the top margin of the service navigation component's mobile toggle button being too small when not used alongside a service name.

packages/govuk-frontend/src/govuk/components/service-navigation/_index.scss

@@ -90,7 +90,7 @@
   .govuk-service-navigation__toggle {
     @include govuk-font($size: 19, $weight: bold);
     display: inline-flex;
-    margin: 0 0 govuk-spacing(2);
+    margin: govuk-spacing(2) 0;
     padding: 0;
     border: 0;
     color: $govuk-service-navigation-link-colour;
@@ -117,6 +117,12 @@
     &[hidden] {
       display: none;
     }
+
+    // If we have both a service name and navigation toggle, remove the
+    // margin-top so that there isn't a bunch of space between them
+    .govuk-service-navigation__service-name + .govuk-service-navigation__wrapper & {
+      margin-top: 0;
+    }
   }
 
   .govuk-service-navigation__list {