Is it possible to make a schedule Ship date default to null value in OM Defaulting rule setup?

Functional user made a request to leave blank for schedule ship date, when it is a copy order. Before booking they will schedule it. By using source type pl/sql api I was able to default to any future date based on ONT_LINE_DEF_HDLR.g_record.source_document_line_id. But I am unable to Return Null value. If the funtion return null, schedule ship date default to sysdate. I am not sure whether it is a doable or not. If you all have any idea, let me know. Thank You Padma