Activity History Display Should Not Draw A Callback Link If the Callback Is Not Defined

Drupal installs frequently use "multihost" installs where multiple Drupal sites reference the same CiviCRM domain and data store. So a module that registers a callback for an activity history entry may not be loaded when an activity history entry displays. In these cases, no link should be drawn, since calling the link via CRM_History_Page_Activity::run() is a guaranteed error.