merged PR zendframework/zf1#680 to fix "Zend_Json::decode null or empty string throw Zend_Json_Exception on PHP7"

Author: falkenhawk

library/Zend/Json.php

@@ -73,6 +73,9 @@ class Zend_Json
      */
     public static function decode($encodedValue, $objectDecodeType = Zend_Json::TYPE_ARRAY)
     {
+        if (in_array($encodedValue, array(null, ''))) {
+            return null;
+        }
         $encodedValue = (string) $encodedValue;
         if (function_exists('json_decode') && self::$useBuiltinEncoderDecoder !== true) {
             $decode = json_decode($encodedValue, $objectDecodeType);