Avoid cache building for super-weird case when repository root_ref is nil

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/workers/project_cache_worker.rb

@@ -7,6 +7,9 @@ class ProjectCacheWorker
     project = Project.find(project_id)
     project.update_repository_size
     project.update_commit_count
-    project.repository.build_cache
+
+    if project.repository.root_ref
+      project.repository.build_cache
+    end
   end
 end