Check project for backlogs module

The hook is called from the OP core's work package controller index
method. For index no specific WP is selected. Thus, we must use the
selected project to check if the backlogs module is activated.

lib/open_project/backlogs/hooks.rb

@@ -53,10 +53,10 @@ module OpenProject::Backlogs::Hooks
     end
 
     def work_packages_overview_attributes(context = {})
-      work_package = context[:work_package]
+      project = context[:project]
       attributes = context[:attributes]
 
-      return unless work_package.backlogs_enabled?
+      return unless project && project.module_enabled?(:backlogs)
 
       attributes << :storyPoints
       attributes << :remainingHours