### 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
is_unit_test = airflow_conf.getboolean('core', 'unit_test_mode')
        while (len(ti_status.to_run) > 0 or len(ti_status.running) > 0) and len(ti_status.deadlocked) == 0:
            self.log.debug("*** Clearing out not_ready list ***")
            ti_status.not_ready.clear()
            
            
            
            
            @provide_session
            def _per_task_process(key, ti: TaskInstance, session=None):
                ti.refresh_from_db(lock_for_update=True, session=session)
                task = self.dag.get_task(ti.task_id, include_subdags=True)
                ti.task = task
                self.log.debug("Task instance to run %s state %s", ti, ti.state)
                
                
                if ti.state == State.SUCCESS:
                    ti_status.succeeded.add(key)
                    self.log.debug("Task instance %s succeeded. Don't rerun.", ti)
                    ti_status.to_run.pop(key)
                    if key in ti_status.running:
                        ti_status.running.pop(key)
                    return
                elif ti.state == State.SKIPPED:
                    ti_status.skipped.add(key)
                    self.log.debug("Task instance %s skipped. Don't rerun.", ti)
                    ti_status.to_run.pop(key)
                    if key in ti_status.running:
                        ti_status.running.pop(key)
                    return