CRM-14120 e-notice error due to returning NULL not by reference, From what I recall php lazy copying means that we don't lose any performance benefit by not returning as a referenct

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* CRM-14120: Notice Notice: Only variable references should be returned by reference in event() (line 122 of /srv/www/womenslibrary.org.uk/sites/all/modules/civicrm/CRM/Event/PseudoConstant.php).
  http://issues.civicrm.org/jira/browse/CRM-14120
Merge pull request #2401 from eileenmcnaughton/CRM-14120

CRM-14120 e-notice error due to returning NULL not by reference,