Do not check datetime_out_of_range if a value is empty.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@14436 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Form/FormulatorPatch.py

@@ -998,11 +998,11 @@ class PatchedDateTimeValidator(DateTimeValidator):
 
         # check if things are within range
         start_datetime = field.get_value('start_datetime')
-        if (start_datetime is not None and
+        if (start_datetime not in (None, '') and
             result < start_datetime):
             self.raise_error('datetime_out_of_range', field)
         end_datetime = field.get_value('end_datetime')
-        if (end_datetime is not None and
+        if (end_datetime not in (None, '') and
             result >= end_datetime):
             self.raise_error('datetime_out_of_range', field)