CRM-13960 - import - ensure empty fields don't throw validation errors

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* CRM-13960: importing an empty custom data value for bolean type results in validation error
  http://issues.civicrm.org/jira/browse/CRM-13960
Merge pull request #2201 from jmcclelland/import-empty-custom-boolean

CRM-13960 - import - ensure empty fields don't throw validation errors