Description:
------------
Prior to PHP 7, the behaviour of an empty string when doing a json_decode() would not cause any error to occur, now with PHP 7 it causes a syntax error to occur.

Test script:
---------------
json_decode("");

if (json_last_error() > 0) {
    echo sprintf("%s (%d)\n", json_last_error_msg(), json_last_error());
}

Expected result:
----------------
No error returned

Actual result:
--------------
Error returned; Syntax error (4)