Array to string conversion in ::processCheckbox

When editing a link, the following check is being made in the menu_link_destination_form_menu_link_content_menu_link_content_form_alter.

This is because the menu_link_destination is stored in an array with

'options' => [
  'menu_link_destination' => $value,
];

The value should be assigned as a default value and not the array.