Issuable#remote_issuable may be nil

Error: https://ops.gitlab.net/gitlab-org/release/tools/-/jobs/2381070

NoMethodError: undefined method `web_url' for nil:NilClass
lib/release_tools/issuable.rb:59:in `url'

Which was raising an error on:

def url
  remote_issuable.web_url
end

That is Issuable#remote_issuable is nil. We can guard this by using Issuable#exists? which checks Issuable#remote_issuable.