cubicweb-brainomics #3344704 __unique_together__ unwhished fonctionnement [deprecated]

When one of field specified in constraint __unique_together__ is empty, the constraint does still not work

Disfonctionnement illustration:
After modifying class Chromosome as below

class Chromosome(EntityType):
"""Chromosome"""
__unique_together__ = [('name', 'identifier')]
name = String(#required=True, unique=True,
maxsize=16)
identifier = String(#required=True,
indexed=True, maxsize=64)

and inserted twice the same no-name chromosome, the table cantains these following records

cw_name | cw_identifier | cw_modification_date | cw_creation_date | cw_cwuri | cw_eid
---------+---------------+----------------------+---------------------+----------+--------
| // | 2013-12-02 00:00:00 | 2013-12-02 00:00:00 | 12345 | 12345
| // | 2013-12-02 00:00:00 | 2013-12-02 00:00:00 | 12346 | 12346
(2 rows)

__unique_together__ seems not work when a field concerned by the constraint is empty
Could you please adress and correct this issue...
(when correction will be done #3347824 could be close too)