checkboxes widget saves extra empty items in the DB

Problem/Motivation

I created a node with a PER field, and set two values on it.

In PHP code, I load the node and iterate the field like this:

    foreach ($node->field_test_per as $value) {
      dsm($value);
    }

This outputs 3 items, instead of the expected 2.

The last item has 'target_id' of 0 and primary of 0.

This is stored in the DB too. I can reproduce this by saving a host node with the checkboxes widget -- a DB row is made in the field table for EVERY item in the widget, not just the selected ones.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes