Do not migrate line items for orders which are not migrated

Tried the patch, but didn't seem to remove all cart line items. Still had order items with null order_id.

I found this to be more dependable:

    // Join the order table and skip all items from orders that are carts.
    $query->leftjoin('commerce_order', 'o', 'o.order_id = li.order_id');
    $query->addField('o', 'status', 'o_status');
    $query->condition('o.status', 'checkout_checkout', 'NOT IN');
    $query->condition('o.status', 'checkout_shipping', 'NOT IN');
    $query->condition('o.status', 'checkout_review', 'NOT IN');
    $query->condition('o.status', 'cart', 'NOT IN');

I ran the new source plugin test without the fix and the test passed. I have not investigated but I am setting to NW.