### 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)

tests/cli/commands/test_dag_command.py
dag_ids = [
            'example_bash_operator',  
            'latest_only',  
            'example_python_operator',  
            'example_xcom',
        ]  
        
        with create_session() as session:
            dr = session.query(DagRun).filter(DagRun.dag_id.in_(dag_ids))
            dr.delete(synchronize_session=False)
        
        args = self.parser.parse_args(['dags', 'next-execution', dag_ids[0]])
        with contextlib.redirect_stdout(io.StringIO()) as temp_stdout:
            dag_command.dag_next_execution(args)
            out = temp_stdout.getvalue()
            
            
            assert "None" in out
        
        now = DEFAULT_DATE
        expected_output = [
            (now + timedelta(days=1)).isoformat(),
            (now + timedelta(hours=4)).isoformat(),
            "None",
            "None",
        ]
        expected_output_2 = [
            (now + timedelta(days=1)).isoformat() + os.linesep + (now + timedelta(days=2)).isoformat(),
            (now + timedelta(hours=4)).isoformat() + os.linesep + (now + timedelta(hours=8)).isoformat(),