### 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/models/dag.py
}
            )
        **See**: `Jinja Environment documentation
        <https://jinja.palletsprojects.com/en/2.11.x/api/#jinja2.Environment>`_
    :type jinja_environment_kwargs: dict
    :param render_template_as_native_obj: If True, uses a Jinja ``NativeEnvironment``
        to render templates as native Python types. If False, a Jinja
        ``Environment`` is used to render templates as string values.
    :type render_template_as_native_obj: bool
    :param tags: List of tags to help filtering DAGs in the UI.
    :type tags: List[str]
    """
    _comps = {
        'dag_id',
        'task_ids',
        'parent_dag',
        'start_date',
        'schedule_interval',
        'fileloc',
        'template_searchpath',
        'last_loaded',
    }
    __serialized_fields: Optional[FrozenSet[str]] = None
    fileloc: str
    """
    File path that needs to be imported to load this DAG or subdag.
    This may not be an actual file on disk in the case when this DAG is loaded
    from a ZIP file or other DAG distribution format.
    """
    def __init__(