CRM-13120 - APIv3 - Don't return "null" from "create" action

 * Split CRM_Core_HTMLInputCoder in two -> CRM_Utils_API_AbstractFieldCoder and CRM_Utils_API_HTMLInputCoder
 * Add CRM_Utils_API_NullOutputCoder (based on AbstractFieldCoder)

----------------------------------------
* CRM-13120: APIv3 returns string "null" after calling "create" with empty string
  http://issues.civicrm.org/jira/browse/CRM-13120