Boolean used as array - interests can be FALSE

Problem/Motivation

Seeing this error happen in the logs semi-frequently:

Uncaught PHP Exception TypeError: "my_module_mailchimp_lists_interest_groups_alter(): Argument #1 ($interests) must be of type array, false given, called in /var/www/prod/csp/site/core/lib/Drupal/Core/Extension/ModuleHandler.php on line 474"

Steps to reproduce

Unsure.

Proposed resolution

This is fairly easy to prevent by doing proper type checking.

This line in mailchimp_lists_process_subscribe_form_choices() uses a null coalescing operator, but that won't do anything if the value is FALSE rather than NULL. I suggest using is_array() instead.

$interests = $choices['interest_groups'] ?? [];

Remaining tasks

User interface changes

API changes

Data model changes