Description:
------------
This is probably related to https://bugs.php.net/bug.php?id=74478.

Using null coalescing operation on an index that does not exist in an SplFixedArray triggers an exception (Index invalid or out of range) while doing the same with isset() works as expected.

See the following snippets:
 * with isset(): https://3v4l.org/s79uA
 * with null coalescing operator: https://3v4l.org/ucSbZ

For some reason, using a class that extends SplFixedArray and overrides method offsetExists() works: https://3v4l.org/5ohF1.

Test script:
---------------
<?php

$foo = new SplFixedArray(2);
$foo[0] = 'bar1';
$foo[1] = 'bar2';

var_dump($foo[0] ?? null);
var_dump($foo[1] ?? null);
var_dump($foo[2] ?? null);


Expected result:
----------------
string(4) "bar1"
string(4) "bar2"
NULL

Actual result:
--------------
string(4) "bar1"
string(4) "bar2"

Fatal error: Uncaught RuntimeException: Index invalid or out of range in /in/ucSbZ:9
Stack trace:
#0 {main}
  thrown in /in/ucSbZ on line 9