Are you running this query within PHPmyAdmin or similar client? If so you can select your users table from the menu, then hit the search tab at the top, in the DOB collum search for blank, then from there you should be able to mass edit them to a generic value.

I could write you a SQL query to run that would do all of this but I don't know how your database is layed out or collum names.

Hope this helps anyway.