Merge branch 'sh-guard-against-nil-sidekiq-metrics-server' into 'master'

Guard against nil Sidekiq metrics server

Closes #41351

See merge request gitlab-org/gitlab-ce!17254

lib/gitlab/metrics/sidekiq_metrics_exporter.rb

@@ -23,7 +23,7 @@ module Gitlab
       end
 
       def stop_working
-        server.shutdown
+        server.shutdown if server
         @server = nil
       end