Setting database field as nullable

Questions
1

3.2:

How to set a database field to allow null values?:

For some reason a database field is being created as not nullable.
I can always go into the DB and change this option, but I want to know how to prevent it from happening on dev/build from the backend code.

Thank you

 private static $db = array(
    'TopArticle' => 'LinkField'
  );