### Apache Airflow version

2.2.0b2 (beta snapshot)

### Operating System

ubuntu 20.04

### Versions of Apache Airflow Providers

n/a

### Deployment

Virtualenv installation

### Deployment details

pip install /path/to/airflow/src

### What happened

Given any DAG initialized with: `schedule_interval=None`

Run `airflow dags test mydagname $(date +%Y-%m-%d)` and get an error:

```
INFO - No run dates were found for the given dates and dag interval.
```

This behavior changed in https://github.com/apache/airflow/pull/15397, it used to trigger a backfill dagrun at the given date.

### What you expected to happen

I expected a backfill dagrun with the given date, regardless of whether it fit into the `schedule_interval`.

If AIP-39 made that an unrealistic expectation, then I'd hope for some way to define unscheduled dags which can still be tested from the command line (which, so far as I know, is the fastest way to iterate on a DAG.).

As it is, I keep changing `schedule_interval` back and forth depending on whether I want to iterate via `astro dev start` (which tolerates `None` but does superfluous work if the dag is scheduled) or via `airflow dags test ...` (which doesn't tolerate `None`).

### How to reproduce

Initialize a DAG with: `schedule_interval=None` and run it via `airflow dags test mydagname $(date +%Y-%m-%d)`

### Anything else

_No response_

### Are you willing to submit PR?

- [ ] 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)

airflow/jobs/backfill_job.py
self.log.debug('Adding %s to not_ready', ti)
                ti_status.not_ready.add(key)
            try:
                for task in self.dag.topological_sort(include_subdag_tasks=True):
                    for key, ti in list(ti_status.to_run.items()):
                        if task.task_id != ti.task_id:
                            continue
                        pool = session.query(models.Pool).filter(models.Pool.pool == task.pool).first()
                        if not pool:
                            raise PoolNotFound(f'Unknown pool: {task.pool}')
                        open_slots = pool.open_slots(session=session)
                        if open_slots <= 0:
                            raise NoAvailablePoolSlot(
                                "Not scheduling since there are "
                                "{} open slots in pool {}".format(open_slots, task.pool)
                            )
                        num_running_task_instances_in_dag = DAG.get_num_task_instances(
                            self.dag_id,
                            states=self.STATES_COUNT_AS_RUNNING,
                            session=session,
                        )
                        if num_running_task_instances_in_dag >= self.dag.max_active_tasks:
                            raise DagConcurrencyLimitReached(
                                "Not scheduling since DAG max_active_tasks limit is reached."
                            )
                        if task.max_active_tis_per_dag:
                            num_running_task_instances_in_task = DAG.get_num_task_instances(
                                dag_id=self.dag_id,
                                task_ids=[task.task_id],
                                states=self.STATES_COUNT_AS_RUNNING,