### 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
"""
        Initializes all components required to run a dag for a specified date range and
        calls helper method to execute the tasks.
        """
        ti_status = BackfillJob._DagRunTaskStatus()
        start_date = self.bf_start_date
        
        dagrun_start_date = timezone.coerce_datetime(start_date)
        if self.bf_end_date is None:
            dagrun_end_date = pendulum.now(timezone.utc)
        else:
            dagrun_end_date = pendulum.instance(self.bf_end_date)
        dagrun_infos = list(self.dag.iter_dagrun_infos_between(dagrun_start_date, dagrun_end_date))
        if self.run_backwards:
            tasks_that_depend_on_past = [t.task_id for t in self.dag.task_dict.values() if t.depends_on_past]
            if tasks_that_depend_on_past:
                raise AirflowException(
                    'You cannot backfill backwards because one or more tasks depend_on_past: {}'.format(
                        ",".join(tasks_that_depend_on_past)