Test time is None to display empty value in tree

issue5230
review15901002

tryton/gui/window/view_form/view/list_gtk/widget.py

@@ -377,7 +377,7 @@ class Time(Date):
         if not record:
             return ''
         value = record[self.attrs['name']].get_client(record)
-        if value:
+        if value is not None:
             return value.strftime(self.renderer.props.format)
         else:
             return ''