various fixes vor php 8.1 compatibility

Cherry-picked from https://git-gitea.tt-rss.org/fox/tt-rss/pulls/56,
while excluding changes in vendor/ (causes compatiblity issues with
PHP<8 [1]) and strftime-related ones (already re-applied in
https://git-gitea.tt-rss.org/fox/tt-rss/pulls/66).

[1] https://community.tt-rss.org/t/support-for-php-8-1/5089/9

classes/pref/system.php

@@ -138,7 +138,7 @@ class Pref_System extends Handler_Administrative {
 					$sth->execute($errno_values);
 
 					while ($line = $sth->fetch()) {
-						foreach ($line as $k => $v) { $line[$k] = htmlspecialchars($v); }
+						foreach ($line as $k => $v) { $line[$k] = htmlspecialchars($v ?? ''); }
 						?>
 						<tr>
 							<td class='errno'>