Segments: filter custom field by 'is empty'/'is not empty'

As a user with recipient lists of varying levels of completeness, I want to create segments filtered simply based on the presence/absence of certain custom fields so that I can target groups that meet certain data quality thresholds.

This could be accomplished by adding two additional filter actions to the ui (with no value required), and updating the segment model to use 'is null/is not null' conditions to the segment query.