Triggering a dagrun with naive datetime raises an HTML page instead of JSON

**Apache Airflow version**:2.0


**What happened**:

An HTML page was returned instead of json when the REST API was triggered with a naive datetime

**What you expected to happen**:

I expected it to raise error status 400 indicating that it's a bad request

**How to reproduce it**:
1. Start the web server and scheduler inside breeze.
2. Make a post request to this endpoint: http://localhost:28080/api/v1/dags/example_bash_operator/dagRuns
 to trigger dagruns using this request body: {"execution_date": "2020-11-09T16:25:56.939143"}. or any naive datetime
3. See that an HTML is raised instead of Json

airflow/api_connexion/endpoints/dag_run_endpoint.py
data["start_date_gte"],
        data["start_date_lte"],
        data["page_limit"],
        data["page_offset"],
    )
    return dagrun_collection_schema.dump(DAGRunCollection(dag_runs=dag_runs, total_entries=total_entries))
@security.requires_access(
    [
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
        (permissions.ACTION_CAN_CREATE, permissions.RESOURCE_DAG_RUN),
    ]
)
@provide_session
def post_dag_run(dag_id, session):
    """Trigger a DAG."""
    if not session.query(DagModel).filter(DagModel.dag_id == dag_id).first():
        raise NotFound(title="DAG not found", detail=f"DAG with dag_id: '{dag_id}' not found")
    post_body = dagrun_schema.load(request.json, session=session)
    dagrun_instance = (
        session.query(DagRun).filter(DagRun.dag_id == dag_id, DagRun.run_id == post_body["run_id"]).first()
    )
    if not dagrun_instance:
        dag_run = DagRun(dag_id=dag_id, run_type=DagRunType.MANUAL, **post_body)
        session.add(dag_run)
        session.commit()
        return dagrun_schema.dump(dag_run)
    raise AlreadyExists(
        detail=f"DAGRun with DAG ID: '{dag_id}' and DAGRun ID: '{post_body['run_id']}' already exists"
    )