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
"""Get a DAG Run."""
    dag_run = session.query(DagRun).filter(DagRun.dag_id == dag_id, DagRun.run_id == dag_run_id).one_or_none()
    if dag_run is None:
        raise NotFound(
            "DAGRun not found",
            detail=f"DAGRun with DAG ID: '{dag_id}' and DagRun ID: '{dag_run_id}' not found",
        )
    return dagrun_schema.dump(dag_run)
@security.requires_access(
    [
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
    ]
)
@format_parameters(
    {
        'start_date_gte': format_datetime,
        'start_date_lte': format_datetime,
        'execution_date_gte': format_datetime,
        'execution_date_lte': format_datetime,
        'end_date_gte': format_datetime,
        'end_date_lte': format_datetime,
        'limit': check_limit,
    }
)
@provide_session
def get_dag_runs(