Support blank string for RAILS_RELATIVE_URL_ROOT

What does this MR do?

Changes our Rackup file to also handle an empty RAILS_RELATIVE_URL_ROOT value.

We want to change the GDK default from / to an empty string because / is an invalid relative url root and causes problems with gems like premailer-rails. See #195514 (closed) details.

We can also change GDK to not set the ENV var at all but it complicates the changes and I think it's still better that we also support an empty string here.

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