Depends on D19126. Ref T13090. For datasource custom fields, this proxies the datasource and provides "none()" and "any()" functions to allow you to search for objects with no values or any values.

• Created a custom "Owning Group" field in Maniphest using a Projects datasource.
• For a task with no owner assigned, searched for "none()" (hit) and "any()" (miss).
• Assigned the task to an owning project.
• Searched for "none()" (miss), "any()" (hit), the project it is now a member of (hit) and some random other project (miss).