Allow paths to components (or classes) in IS (not) NULL statement and
not only terminal properties. As long as component types are concerned,
testing "is null" corresponds to testing each column of the component
(or each column of the key for a class), based on the values returned
by the nullSafeSet(null) on the component or key type.
This would allow usage of multi-column ids, as for now these cannot
be used in HQL (cannot name each column as in composite-id).