Steps to reproduce the issue

Access the Joomla instance when there is no user agent in the request

Expected result

No PHP error, warning or notice

Actual result

PHP Deprecated: stripos(): Passing null to parameter #1 ($haystack) of type string is deprecated in libraries/vendor/joomla/application/src/Web/WebClient.php on line 286

System information (as much as possible)

Joomla version 4.3.2
PHP Version 8.1.21

Additional comments

Causes an issue in my development of an appliance that accesses Joomla in raw format via curl seeking image data. The notice corrupts the image data. I would rather not do development work without error reporting.