Undefined variable $addresses AddressAdjusterBase.php:38

Addition: Removing the recipients to prevent sending updates was the only way when Drupal used swiftmailer, as the update manager only has two options for the email notification threshold: all updates or only security updates, but no option to not send a notification.

After applying the MR on my site and looking into the policy I found the "skip sending" adjuster which is the correct way of not sending notifications on the test environment. But still there might be other use cases where the recipients are evaluated dynamically (or defined by users) and can be none, so the code still should handle that properly.

I believe the steps to reproduce are wrong. If you want no recipients then you don't add the adjuster. If you add the adjuster but don't set any address, then you get an error like "You must set at least one To address." On your site where you have the problem, then if you tried to edit the policy then you would also see the same error.

The address adjuster should never have config without any addresses, and it enforces this. If you edit the config programmatically to create illegal config then that is not the fault of this module.

Skip sending is entirely different - it blocks the email entirely which is not the same as having no "To" address (in which case you could still have a "Bcc").