Steps to reproduce the issue

Do a raw http request on an URL that gets redirected, like / on a multi-language site:
$ openssl s_client -connect some-website.com:443
GET / HTTP/1.1[enter]
Host: some-website.com[enter]
[enter]

Expected result

No error

Actual result

PHP Deprecated: stripos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /htdocs/libraries/vendor/joomla/application/src/Web/WebClient.php on line 380 (and other lines)

System information

php8.3-fpm 8.3.6-1+020240424.32+debian121.gbp9a7ce5

Additional comments

I don't know which client actually does http request without user agent, but I see these errors in the logs quite often.
Anyway, we should always validate user-supplied variables.

An easy fix should be:
htdocs/libraries/vendor/joomla/application/src/Web/WebClient.php
[...]
public const NONE = 0; // maybe UNKOWN instead?
[...]
protected function detectEngine($userAgent)
{
if(empty(trim($userAgent))) {
$this->engine = self::NONE;
} elseif [...]
}