CRM-13027 - CRM_Core_BAO_CustomField - Re-allow blank $customDataType

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* CRM-13027: Default getFields returns irrelevant custom-data fields
  http://issues.civicrm.org/jira/browse/CRM-13027
Merge pull request #1565 from totten/master-getfields-fix2

CRM-13027 - CRM_Core_BAO_CustomField - Re-allow blank $customDataType