Prevent upgrade "Undefined property $reverseproxyignore" notice

This is a fix for a little regression introduced @ MDL-67861. Applies to all current branches (35 and up).

When these conditions happen:

• The site has debugging set to high levels + display errors enabled.
• The site has the handling of X-Forwarded-For headers enabled.
• There are some headers coming in the requests.
• The site is upgraded via web (CLI is immune to this).

This notice is shown in the initial 3-4 pages (login, env, db upgrade...) until the new settings are saved to DB (then the notice disappears forever):

Notice: Undefined property: stdClass::$reverseproxyignore in /opt/app/lib/moodlelib.php on line 9210"

So, this issue is about to avoid that notice to be shown, just that.

In the original issue, a simple change was proposed, FYC:

Maybe we can just change:

return !\core\ip_utils::is_ip_in_subnet_list($ip, $CFG->reverseproxyignore, ',');

by

return !\core\ip_utils::is_ip_in_subnet_list($ip, $CFG->reverseproxyignore ?? '', ',');

And that's all. Note the notice doesn't lead to any malfunctioning, it's just "ugly" for the sites matching all the conditions above.

CORRECTION TAKE#2 : Striking-through the previous -CORRECTION: it breaks login, so makes it imossible to upgrade sites through the Web UI.-

Ciao