Describe the bug
PHP 8.1 error - PHP Deprecated: htmlspecialchars() is triggered when opening feed settings.

[Sat Jun 11 16:36:47.643857 2022] [proxy_fcgi:error] [pid 25292:tid 140616258250304] [client x.x.x.x:58381] AH01071: Got error 'PHP message: PHP Deprecated: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in /home/ubuntu/FreshRSS/lib/Minz/Helper.php on line 20'

To Reproduce
Steps to reproduce the behavior:

1. Go to Subscription management page
2. Click on the cog next to any feed
3. Check the PHP error log for the message

Expected behavior
No error.