Warning: Undefined array key "display_empty"

Thanks for reporting this, the proposed solution is good, it’s likely coming from an upgrade where we didn’t provide defaults in a hook or otherwise

Kindly review

IMHO, the real solution would be to update the menu_block.post_update.php file with a new function to add the new key for sites that installed the module prior to the new key existing.

See https://git.drupalcode.org/project/menu_block/-/blob/8.x-1.x/menu_block.... for an extremely similar example.