### 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
'dags',
                        'backfill',
                        'example_bash_operator',
                        '--task-regex',
                        'runme_0',
                        '--dry-run',
                        '--start-date',
                        DEFAULT_DATE.isoformat(),
                    ]
                ),
                dag=dag,
            )
        output = stdout.getvalue()
        assert f"Dry run of DAG example_bash_operator on {DEFAULT_DATE.isoformat()}\n" in output
        assert "Task runme_0\n" in output
        mock_run.assert_not_called()  
        dag_command.dag_backfill(
            self.parser.parse_args(
                [
                    'dags',
                    'backfill',
                    'example_bash_operator',
                    '--dry-run',
                    '--start-date',
                    DEFAULT_DATE.isoformat(),
                ]
            ),
            dag=dag,
        )
        mock_run.assert_not_called()