Several empty Contact fields are set to the "null" instead of NULL

When I use the Drupal Devel module to dpm() a new Contact (DAO) after editing, I see that many empty fields are NULL while some are "null" e.g. nick_name and external_identifier.
This might be a problem when someone named "Johnny Null Jones" comes along.
Also, if I enter a new Contact e.g. "James Null", then when I edit the Contact, the last name field is empty, forcing you to enter the last name of "Null" again.
This applies to several but not all Contact fields which are not part of the name.

Like it or not there are actually many people named "Null" in the world.
See http://www.411.com/dir/a-z/null

I set this to Minor, and not Major, only because it's the first issue I am reporting here.