Fix redirect loop when relative url root is blank

Sets the cookie path to `/` if RAILS_RELATIVE_URL_ROOT is set to an
empty string

Changelog: fixed

config/initializers/session_store.rb

@@ -27,5 +27,5 @@ Gitlab::Application.config.session_store(
   secure: Gitlab.config.gitlab.https,
   httponly: true,
   expires_in: Settings.gitlab['session_expire_delay'] * 60,
-  path: Rails.application.config.relative_url_root.nil? ? '/' : Gitlab::Application.config.relative_url_root
+  path: Rails.application.config.relative_url_root.presence || '/'
 )