[PropertyAccess] Fix nullsafe operator on array index

[HypeMC]

Q	A
Branch?	6.2
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	-
License	MIT
Doc PR	-

Currently the nullsafe operator doesn't work with array index paths. However, this is not obvious at first since THROW_ON_INVALID_PROPERTY_PATH is not enabled by default:

use Symfony\Component\PropertyAccess\PropertyAccessor;

var_dump((new PropertyAccessor())->getValue([], '[foo]')); // NULL
var_dump((new PropertyAccessor())->getValue([], '[foo?]')); // NULL

var_dump((new PropertyAccessor(throw: PropertyAccessor::THROW_ON_INVALID_INDEX))->getValue([], '[foo]'));
// Cannot read index "foo" while trying to traverse path "[foo]".

var_dump((new PropertyAccessor(throw: PropertyAccessor::THROW_ON_INVALID_INDEX))->getValue([], '[foo?]'));
// Cannot read index "foo" while trying to traverse path "[foo]". (THIS IS WRONG, SHOULD BE NULL)

[nicolas-grekas]

Thank you @HypeMC.