Allow all models for reference if there is no restriction

If the allowed models is empty, it means that there is no constraint on the
reference model. So we must allow all of them.

issue8300
review275311002

tryton/common/selection.py

@@ -93,7 +93,7 @@ class SelectionMixin(object):
 
         def _model_evaluator(allowed_models):
             def test(value):
-                return value[0] in allowed_models
+                return value[0] in allowed_models or not allowed_models
             return test
 
         if field.attrs['type'] == 'reference':