Add missing _default for TimeField

issue2940
review631003
(transplanted from 034e965007685e470598698fec3a235d2df3bcb3)

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

@@ -256,6 +256,8 @@ class DateField(CharField):
 
 class TimeField(CharField):
 
+    _default = None
+
     def set_client(self, record, value, force_change=False):
         if not isinstance(value, datetime.time):
             try: