### 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/jobs/backfill_job.py
session=session,
                            )
                            if num_running_task_instances_in_task >= task.max_active_tis_per_dag:
                                raise TaskConcurrencyLimitReached(
                                    "Not scheduling since Task concurrency limit is reached."
                                )
                        _per_task_process(key, ti)
            except (NoAvailablePoolSlot, DagConcurrencyLimitReached, TaskConcurrencyLimitReached) as e:
                self.log.debug(e)
            self.heartbeat(only_if_necessary=is_unit_test)
            
            executor.heartbeat()
            
            
            
            if (
                ti_status.not_ready
                and ti_status.not_ready == set(ti_status.to_run)
                and len(ti_status.running) == 0
            ):
                self.log.warning("Deadlock discovered for ti_status.to_run=%s", ti_status.to_run.values())
                ti_status.deadlocked.update(ti_status.to_run.values())
                ti_status.to_run.clear()
            
            self._manage_executor_state(ti_status.running)
            
            self._update_counters(ti_status=ti_status)
            
            _dag_runs = ti_status.active_runs[:]
            for run in _dag_runs: