[5.x] Fix deprecated: third parameter of preg_replace() must not be null

[miloslavkostir]

Statamic\Search\Comb\Comb::flattenArray($item, $glue = ' '):
The parameter $item is mixed (null as well). The third parameter of preg_replace() must be array|string. PHP 8.1+ triggers a deprecated error if $item is null.

// If $item is not array...
if (! is_array($item)) {
    // ...it must be string
    return preg_replace('#\s+#ism', ' ', (string) $item);
}

Test script (does not work in Tests\Search\CombTest):

<?php

use Statamic\Search\Comb\Comb;

require_once __DIR__ . '/vendor/autoload.php';

ini_set('error_reporting', E_ALL);

$comb = new Comb([
    ['title' => 'John Doe', 'email' => null],
    ['title' => 'Jane Doe', 'email' => '[email protected]'],
]);

var_dump($comb->lookUp('John'));