Appending null produces null?

extracts=# select bill_address1, bill_address2, bill_address1 || ' ' ||
bill_address2 from customers where external_id = 20037514;
  bill_address1  | bill_address2 | ?column?
-----------------+---------------+----------
 6745 Avalon Ave |               |
(1 row)

I know that the usual operations with 'null' will produce null, but I didn't
expect it in the case of a text field being appended to another text field.
No doubt that follows as a matter of strict principle, but the outcome makes
no sense from a purely practical point of view.  Is this really the intended
behaviour?  I know that I can work around this with a 'coalesce' or
something similar, but that seems unnecessarily messy.