When a user leaves a Form Input Box or Text Area blank, what should I store in my database?

Conventional wisdom would say a NULL since they never - in this scenario - filled out the field.

I believe that for Inputs and TextAreas, HTML inserts an Empty String into the $_POST array, so by default you would be writing Empty Strings into your database.

Right now I am checking all of my Form Validation, and I want my PHP to insert the *proper* data-type into fields whether that should be a NULL, Empty String, or 0.