Description:
------------
When I use null coalescing operator on a class implementing SplFixedArray then offsetExists is not called, but offsetGet is called instead (which may trigger some error). It should be equivalent to ternary operator with isset.

This is similar to https://bugs.php.net/bug.php?id=71731 (here - SplFixedArray, there - ArrayAccess).

https://3v4l.org/#v714


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

$arrayData = new class implements \ArrayAccess
{
	public function offsetExists($offset)
	{
		echo "offsetExists\n";
	}


	public function offsetGet($offset)
	{
		echo "offsetGet\n";
	}


	public function offsetSet($offset, $value)
	{
	}


	public function offsetUnset($offset)
	{
	}
};
echo "arrayData - isset:\n";
isset($arrayData['foo']) ? NULL : NULL;
echo "arrayData - null coalescing operator:\n";
$arrayData['foo'] ?? NULL;


$fixedData = new class extends \SplFixedArray
{
	public function offsetExists($offset)
	{
		echo "offsetExists\n";
	}


	public function offsetGet($offset)
	{
		echo "offsetGet\n";
	}


	public function offsetSet($offset, $value)
	{
	}


	public function offsetUnset($offset)
	{
	}

};
echo "fixedData - isset:\n";
isset($fixedData['foo']) ? NULL : NULL;
echo "fixedData - null coalescing operator:\n";
$fixedData['foo'] ?? NULL;

Expected result:
----------------
arrayData - isset:
offsetExists
arrayData - null coalescing operator:
offsetExists
fixedData - isset:
offsetExists
fixedData - null coalescing operator:
offsetExists

Actual result:
--------------
arrayData - isset:
offsetExists
arrayData - null coalescing operator:
offsetExists
fixedData - isset:
offsetExists
fixedData - null coalescing operator:
offsetGet