Add one method to base event fetcher

FIx file

lib/gitlab/cycle_analytics/base_event_fetcher.rb

@@ -70,6 +70,10 @@ module Gitlab
       def allowed_ids_source
         { project_id: @project.id }
       end
+
+      def projects
+        [@project]
+      end
     end
   end
 end

lib/gitlab/cycle_analytics/usage_data.rb

@@ -33,6 +33,7 @@ module Gitlab
       def medians_per_stage
         projects.each_with_object({}) do |project, hsh|
           ::CycleAnalytics::ProjectLevel.new(project: project, options: options).all_medians_per_stage.each do |stage_name, median|
+            hsh[stage_name] ||= []
             hsh[stage_name] << median
           end
         end