[PHP-INTERNALS] fix bug #53432 (Assignment via string index access on an empty string converts to array)

Hi all,

This PR seems good fix for 5.6.

https://github.com/php/php-src/pull/463

Test script:
---------------
$a = ''; // empty string
$a[10] = 'a';
echo $a; // "Array"

$b = ' '; // non empty string
$b[10] = 'b';
echo $b; // " b"

Expected result:
----------------
" a"
" b"

Actual result:
--------------
"Array"
" b"

Current behavior does not make sense.

Regards,