Empty corename on acquia_override_subscription_corename config

Under some cases, acquia_override_subscription_corename in the environment's configuration will be empty, and this can trigger a state where the identifier in the Solr URL is empty, like this:

http://useast1-c1.acquia-search.com/solr/

This patch hardens the code to avoid setting the URL to having an empty identifier like the above.