- the allow_null_pks flag on mapper() is deprecated, and

- the allow_null_pks flag on mapper() is deprecated, and
the feature is turned "on" by default.  This means that
a row which has a non-null value for any of its primary key
columns will be considered an identity.  The need for this
scenario typically only occurs when mapping to an outer join.
[ticket:1339]
- streamlined the NULL check to use set operations