Merge branch 'backport/groups-helper-refactor' into 'master'

Backport groups_helper refactor from gitlab-org/gitlab-ee!904

This is a simple backport from a change in `groups_helper.rb` introduced by gitlab-org/gitlab-ee!904, requested here: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/904#note_18894726

See merge request !7767

app/helpers/groups_helper.rb

@@ -8,11 +8,7 @@ module GroupsHelper
       group = Group.find_by(path: group)
     end
 
-    if group && group.avatar.present?
-      group.avatar.url
-    else
-      image_path('no_group_avatar.png')
-    end
+    group.try(:avatar_url) || image_path('no_group_avatar.png')
   end
 
   def group_title(group, name = nil, url = nil)