Bring back Sidekiq class constant check

This should always be safe, since `Sidekiq.server?` would never
succeed without this to exist, and it's more consistent.

lib/gitlab/runtime.rb

@@ -38,7 +38,7 @@ module Gitlab
       end
 
       def sidekiq?
-        !!Sidekiq.server?
+        !!(defined?(::Sidekiq) && Sidekiq.server?)
       end
 
       def console?