[entity] fix crash when using cw_instantiate with empty related entity list; closes #2094462

--HG--
branch : stable

entity.py

@@ -481,7 +481,9 @@ class Entity(AppObject):
             assert eschema.has_relation(attr, role), '%s %s not found on %s' % (attr, role, eschema)
             rschema = eschema.subjrels[attr] if role == 'subject' else eschema.objrels[attr]
             if not rschema.final and isinstance(value, (tuple, list, set, frozenset)):
-                if len(value) == 1:
+                if len(value) == 0:
+                    continue # avoid crash with empty IN clause
+                elif len(value) == 1:
                     value = iter(value).next()
                 else:
                     # prepare IN clause

test/unittest_entity.py

@@ -143,6 +143,8 @@ class EntityTC(CubicWebTC):
         req = self.request()
         p1 = req.create_entity('Personne', nom=u'di')
         p2 = req.create_entity('Personne', nom=u'mascio')
+        t = req.create_entity('Tag', name=u't0', tags=[])
+        self.assertItemsEqual(t.tags, [])
         t = req.create_entity('Tag', name=u't1', tags=p1)
         self.assertItemsEqual(t.tags, [p1])
         t = req.create_entity('Tag', name=u't2', tags=p1.eid)