Describe the bug
Deprecation warnings are reported.

I'm using current master, commit 28cff8a with PHP 8.1.0.

PHP Deprecated:  strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /var/www/localhost/htdocs/FreshRSS/app/Models/Feed.php on line 408

To Reproduce
Using PHP 8.1, run actualize_script.php

Expected behavior
No deprecation or other errors should be reported.

Screenshots
n/a

Environment information (please complete the following information):

• Device: Linux
• OS: Linux
• Browser: n/a
• FreshRSS version: current master, 28cff8a
• Database version: MariaDB 10.6.5
• PHP version: 8.1.0
• Installation type: self hosted

Additional context
I thought I had seen all the deprecation warnings in #4010 but I didn't notice this one as it only happens sometimes, I'm guessing only when new items appear in the feed.