Support blank string for RAILS_RELATIVE_URL_ROOT

GDK defaulted to `/` but that has issues with some gems because that is
not a valid relative root so we want to change the GDK default to an
empty string.

This changes our rackup file to handle a blank string properly.

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