Schema: syntax to filter out blank fields

Is there a folder creation syntax to filter out content where a field is blank?

For example, in our asset.yml which connects to Asset entity type:

filters:
 - { "path": "project", "relation": "is", "values": [ "$project" ] }
 - { "path": "assets", "relation": "is_not", "values": [ "blank" ] }

where assets is a multi-entity link to Asset representing subassets and could have blank fields. Is this possible and what should we put for “values”?

This works in the UI if you have no value so does an empty list not work?

filters:
 - { "path": "project", "relation": "is", "values": [ "$project" ] }
 - { "path": "assets", "relation": "is_not", "values": [] }

Using null worked for me. Thanks!

filters:
 - { "path": "project", "relation": "is", "values": [ "$project" ] }
 - { "path": "assets", "relation": "is_not", "values": [ null ] }