Fix "array offset on value of type null" for $error and $old_error

I tried applying to only $error and only $old_error, but both appear to be needed.

Log entries:
E_NOTICE (8) 	classes/urlhelper.php:464 	Trying to access array offset on value of type null
1. classes/urlhelper.php(464): ttrss_error_handler(8, Trying to access array offset on value of type null, classes/urlhelper.php, 464, [)
2. classes/rssutils.php(464): fetch([{"url":"https://some.url.rss","login":"","pass":"","timeout":15,"last_modified":"Sat, 31 Aug 2019 15:22:31 GMT"})
3. update.php(235): update_rss_feed(732, 1)

classes/urlhelper.php

@@ -461,7 +461,7 @@ class UrlHelper {
 			if (self::$fetch_last_error_code != 200) {
 				$error = error_get_last();
 
-				if ($error['message'] != $old_error['message']) {
+				if (($error['message'] ?? '') != ($old_error['message'] ?? '')) {
 					self::$fetch_last_error .= "; " . $error["message"];
 				}