### 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
return qry.scalar()
    @classmethod
    def get_serialized_fields(cls):
        """Stringified DAGs and operators contain exactly these fields."""
        if not cls.__serialized_fields:
            cls.__serialized_fields = frozenset(vars(DAG(dag_id='test')).keys()) - {
                'parent_dag',
                '_old_context_manager_dags',
                'safe_dag_id',
                'last_loaded',
                'user_defined_filters',
                'user_defined_macros',
                'partial',
                'params',
                '_pickle_id',
                '_log',
                'is_subdag',
                'task_dict',
                'template_searchpath',
                'sla_miss_callback',
                'on_success_callback',
                'on_failure_callback',
                'template_undefined',
                'jinja_environment_kwargs',
                
                'has_on_success_callback',
                'has_on_failure_callback',
            }
        return cls.__serialized_fields
    def get_edge_info(self, upstream_task_id: str, downstream_task_id: str) -> EdgeInfoType: