make the singleton value group by shipped with the engine actually work

app/models/report/group_by/singleton_value.rb

@@ -1,5 +1,10 @@
 class Report::GroupBy
   class SingletonValue < Base
     dont_display!
+
+    def define_group(sql)
+      sql.select "1 as singleton_value"
+      sql.group_by "singleton_value"
+    end
   end
 end