Merge branch 'puma-support-blank-relative-url-root' into 'master'

Support blank string for RAILS_RELATIVE_URL_ROOT

See merge request gitlab-org/gitlab!65974

config.ru

@@ -13,7 +13,7 @@ warmup do |app|
   client.get('/')
 end
 
-map ENV['RAILS_RELATIVE_URL_ROOT'] || "/" do
+map ENV['RAILS_RELATIVE_URL_ROOT'].presence || "/" do
   use Gitlab::Middleware::ReleaseEnv
   run Gitlab::Application
 end