Only cache the event for the fork-source when it exists

It is possible for a fork not to have a source anymore, in this case
we don't need to keep track of the push event.

app/services/users/last_push_event_service.rb

@@ -16,8 +16,8 @@ module Users
         user_cache_key
       ]
 
-      if event.project.forked?
-        keys << project_cache_key(event.project.forked_from_project)
+      if forked_from = event.project.forked_from_project
+        keys << project_cache_key(forked_from)
       end
 
       keys.each { |key| set_key(key, event.id) }

changelogs/unreleased/bvl-fix-push-event-service-for-forks.yml

@@ -0,0 +1,5 @@
+---
+title: Only cache last push event for existing projects when pushing to a fork
+merge_request: 14989
+author:
+type: fixed

spec/services/users/last_push_event_service_spec.rb

@@ -22,7 +22,6 @@ describe Users::LastPushEventService do
     it 'caches the event for the origin project when pushing to a fork' do
       source = build(:project, id: 5)
 
-      allow(project).to receive(:forked?).and_return(true)
       allow(project).to receive(:forked_from_project).and_return(source)
 
       expect(service).to receive(:set_key)