Fix notices thrown when using quick search widget for Employer in contact edit form

Notice: Undefined property: CRM_Core_DAO::$sort_name in civicrm_api3_contact_quicksearch() (line 759 of /var/www/d7.demo.civicrm.org/svn/civicrm_stable/api/v3/Contact.php).

and

Undefined index on results['values'] when input widget is empty.