Bug: Third Flags needs default

[John-Betong]

Info
I have been trying for over a week to trace a new site's "Network connection Error" from going down overnight. No errors in logs so added "strict_types=1" to all PHP app and system PHP files, also set .env -> CI_ENVIRONMENT = development and eventually managed to get some errors reported.

Describe the bug
incorrect type being passed.

CodeIgniter 4 version
CI_VERSION 4.1.1

Affected module(s)
File: ./system/Validation/FormatRules.php
[code]
public function valid_ip(string $ip = null, string $which = null): bool
{
if (empty($ip))
{
return false;
}
switch (strtolower((string) $which))
{
case 'ipv4':
$which = FILTER_FLAG_IPV4;
break;
case 'ipv6':
$which = FILTER_FLAG_IPV6;
break;
default:
# PROBLEM 2021-03-06 - forced (int)
$which = (int) null;
break;
}

	return (bool) filter_var($ip, FILTER_VALIDATE_IP, $which) || (! ctype_print($ip) && (bool) filter_var(inet_ntop($ip), FILTER_VALIDATE_IP, $which));
}

[/code]

Expected behavior, and steps to reproduce if appropriate
I did not expect the web page to crash :(

Context

OS: [Ubuntu 20.04.1]
Web server [Apache 2]
PHP version [8.0.3]

[MGatner]

Since this is essentially the same issue I am closing this and consolidating the other PRs.