7.0.3 and 7.1.3 different results?

A query such as "select sum(pages) from job_documents where
delivery_type='print'" returned 0 in version 7.0.3 if there were no rows
matching the query.  In 7.1.3 the result is NULL if no rows match the
query.  Why the change?  Which result is "correct" according to the SQL
standard?

I'm running PostgreSQL 7.1.3 on FreeBSD 4.3 (x86).

TIA.