When reading a SC.DateTime record attribute, the convertion of a null value, should produce null in all cases, not only for non unix time attributes.

A null value should never be converted to 0 milliseconds because null should remain null in all cases. The alternative (unacceptable) would be to handle this special case (milliseconds=0 i.e. 1.1.1970) into the from method of the same transformer. The previous version was properly handling null only on non useUnixTime attributes.