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 list of DAG Runs"""
    body = request.get_json()
    try:
        data = dagruns_batch_form_schema.load(body)
    except ValidationError as err:
        raise BadRequest(detail=str(err.messages))
    appbuilder = current_app.appbuilder
    readable_dag_ids = appbuilder.sm.get_readable_dag_ids(g.user)
    query = session.query(DagRun)
    if data.get("dag_ids"):
        dag_ids = set(data["dag_ids"]) & set(readable_dag_ids)
        query = query.filter(DagRun.dag_id.in_(dag_ids))
    else:
        query = query.filter(DagRun.dag_id.in_(readable_dag_ids))
    dag_runs, total_entries = _fetch_dag_runs(
        query,
        data["end_date_gte"],
        data["end_date_lte"],
        data["execution_date_gte"],
        data["execution_date_lte"],