CRM-13139 - CRM_Utils_REST - Accept empty JSON objects ("{}")

Note: When the "json" parameter includes malformed JSON (eg "{") or an empty
string (""), the behavior is unchanged. This only affects a well-formed empty-obect.

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* CRM-13139: REST API: json_decode() fails for empty input
  http://issues.civicrm.org/jira/browse/CRM-13139