### 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
self,
        dag_id: str,
        description: Optional[str] = None,
        schedule_interval: ScheduleIntervalArg = ScheduleIntervalArgNotSet,
        timetable: Optional[Timetable] = None,
        start_date: Optional[datetime] = None,
        end_date: Optional[datetime] = None,
        full_filepath: Optional[str] = None,
        template_searchpath: Optional[Union[str, Iterable[str]]] = None,
        template_undefined: Type[jinja2.StrictUndefined] = jinja2.StrictUndefined,
        user_defined_macros: Optional[Dict] = None,
        user_defined_filters: Optional[Dict] = None,
        default_args: Optional[Dict] = None,
        concurrency: Optional[int] = None,
        max_active_tasks: int = conf.getint('core', 'max_active_tasks_per_dag'),
        max_active_runs: int = conf.getint('core', 'max_active_runs_per_dag'),
        dagrun_timeout: Optional[timedelta] = None,
        sla_miss_callback: Optional[Callable[["DAG", str, str, List[str], List[TaskInstance]], None]] = None,
        default_view: str = conf.get('webserver', 'dag_default_view').lower(),
        orientation: str = conf.get('webserver', 'dag_orientation'),
        catchup: bool = conf.getboolean('scheduler', 'catchup_by_default'),
        on_success_callback: Optional[DagStateChangeCallback] = None,
        on_failure_callback: Optional[DagStateChangeCallback] = None,
        doc_md: Optional[str] = None,