CRM-14008 alter update greeting to update blanks as well as null

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* CRM-14008: Addressee greetings not being updated when they are empty
  http://issues.civicrm.org/jira/browse/CRM-14008
Merge pull request #2236 from eileenmcnaughton/CRM-14008

CRM-14008 alter update greeting to update blanks as well as null