Compute timestamp only if value is defined

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@44294 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/Formulator/Widget.py

@@ -1570,6 +1570,7 @@ class DateTimeWidget(Widget):
     if not value and field.get_value('default_now'):
       value = DateTime()
     text_node = Element('{%s}%s' % (TEXT_URI, local_name), nsmap=NSMAP)
+    if value:
       # http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/#dateTime
       attr_dict['{%s}date-value' % OFFICE_URI] = value.ISO8601()
       # According http://wiki.services.openoffice.org/wiki/Documentation/How_Tos/Calc:_Date_%26_Time_functions