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)