Convert UserReferenceFilter#link_to_group to use a guard clause

lib/gitlab/markdown/user_reference_filter.rb

@@ -80,15 +80,13 @@ module Gitlab
       end
 
       def link_to_group(group, namespace)
-        if user_can_reference_group?(namespace)
+        return unless user_can_reference_group?(namespace)
+
         push_result(:user, *namespace.users)
 
         url = urls.group_url(group, only_path: context[:only_path])
 
         %(<a href="#{url}" class="#{link_class}">@#{group}</a>)
-        else
-          nil
-        end
       end
 
       def link_to_user(user, namespace)