Any example of a necessary nullable foreign key?

Question

Customers
 customer_id

Orders
 order_id
 customer_id fk

If I have two tables and define a foreign key on customer_id in the Orders table, by allow

Answer 1

there is, make some tree structure, a table which related to itself. Consider this:

table_node(node_id, parent_node_id, name)

For the root, parent_node_id should be null, right?