GDK generates `relative_url_root` of ` ` instead of `""`

With the change in 15c793db, the generated config is:

relative_url_root: instead of relative_url_root: ""

We have relative_url_root: <%= env_or_default('relative_url_root', '""') %> and

def env_or_default(key, default)
  ENV[key] || default
end

But the ENV variable here is actually a blank string (not nil) causing it not to set the default value of "".

This causes problems like: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27635 https://gitlab.com/gitlab-org/gitlab-ce/issues/61099

Because when the config in the yml is nil, ENV['RAILS_RELATIVE_URL_ROOT'] is used and this is set to / in https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/run#L23

Maybe we should fix that too and make the default "" instead.