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?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start with the Task Instances list and its filter controls, then trace how their filters reach the underlying database query. Compare the proposed empty-value controls with the COALESCE approach and confirm the expected behavior for None-state tasks. Done means users can reliably filter only tasks whose state is None.

Written by the indexing model from the issue text.