not-required xinha fields give issue after upgrade

The 2.09 CMS validates content by checking that the values of mandatory string properties are not empty. This validation was not enabled before, in the 2.06 series. Fill in these fields during the upgrade, such that
a) each document can be saved without errors
b) new documents can be saved directly, without errors

Solution: Do not execute validation for a non-required xinha field. I do not think that a and b are needed - but make sure.