[11.x] Add phpstan assertions for last in Collection isEmpty and isNotEmpty

[bastien-phi]

#51998 and #52184 added PHPStan assertions for Collection::first() in Collection::isEmpty() and Collection::isNotEmpty()

This PR adds the same for Collection::last() as it would be very useful

/**
 * @var Collection<int, string>
 */
$collection = new Collection(['laravel']);

if ($collection->isNotEmpty()) {
    // before this PR:
    \PHPStan\dumpType($collection->last()); // Dumped type: string|null
    
    // after this PR:
    \PHPStan\dumpType($collection->last()); // Dumped type: string
}

[bastien-phi]

Thanks !