Empty dict value should always be an empty dictionary

The client should not detect as changed a Dict field with None as value.

issue4231
review11531002

tryton/gui/window/view_form/model/field.py

@@ -884,6 +884,9 @@ class DictField(Field):
 
     _default = {}
 
+    def get(self, record):
+        return super(DictField, self).get(record) or self._default
+
     def get_client(self, record):
         return super(DictField, self).get_client(record) or self._default