Do not write creation values in ModelSingleton

(grafted from 6ba6ad79208e3aa6af62a480f977f30e2a1f64ae)

--HG--
branch : 6.0

trytond/trytond/model/modelsingleton.py

@@ -58,7 +58,9 @@ class ModelSingleton(ModelStorage):
         singleton = cls.get_singleton()
         if not singleton:
             singleton, = cls.create([values])
-        actions = (records, values) + args
+            actions = (records, {}) + args
+        else:
+            actions = (records, values) + args
         args = []
         for values in actions[1:None:2]:
             args.extend(([singleton], values))