Possibility to filter tasks in `None` state

Description

Currently in list of Task Instances there is no possibility to quickly filter only tasks with None state. This issue proposes to introduce such possibility. I see two approaches that could be introduced to handle this:

• add Is empty and Is not empty filtering options in filter controls
• when querying underlying database to get list of tasks, do COALESCE(state, 'None') (pseudocode) so if user enters None into (for example) Starts with clause, it's filtered properly.

Use case/motivation

None is a valid status for task and it should be possible to filter by it.

Related issues

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct