Description:
------------
If you decode an empty json value, the internal error state doesn't change.

Decode a valid json value and after that an empty value, the error state is 0

Decode an invalid json value and after that an empty value, the error state remains 4 (JSON_ERROR_SYNTAX) (I think it's the same for all error states, but haven't actually tested)

I'm not quite sure whether an empty string is valid json and should reset the error state to 0 or whether it is invalid json and should set the error state to 4. Because a valid empty json string is '""' and not ''

Test script:
---------------
<?php
error_reporting(E_ALL);

json_decode('{"test":"test"}');
var_dump(json_last_error());

json_decode("");
var_dump(json_last_error());


json_decode("invalid json");
var_dump(json_last_error());


json_decode("");
var_dump(json_last_error());

?>

Expected result:
----------------
int(0)
int(0)
int(4)
int(0)


OR

int(0)
int(4)
int(4)
int(4)


Actual result:
--------------
int(0)
int(0)
int(4)
int(4)