Guard against nil Sidekiq metrics server
Shutting down Sidekiq with the Prometheus metrics server would lead to ugly errors:
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/metrics/sidekiq_metrics_exporter.rb:26:in `stop_working': undefined method `shutdown' for nil:NilClass (NoMethodError)
from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/daemon.rb:43:in `block in stop'
from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/daemon.rb:40:in `synchronize'
from /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/daemon.rb:40:in `stop'
Closes #41351 (closed)